refactor(sampler): extract shared utilities to reduce duplication

- Add sampler/common.rs with distribution helpers (internal_bounds,
  from_internal, to_internal, sample_random) used by 8 samplers
- Add sampler/tpe/common.rs with TPE sampling functions
  (sample_tpe_float, sample_tpe_int, sample_tpe_categorical)
  shared by TpeSampler, MultivariateTpeSampler, and MotpeSampler
- Remove ~940 lines of near-identical code across sampler modules

src/sampler/cma_es.rs

@@ -77,6 +77,8 @@ use crate::param::ParamValue;
 use crate::rng_util;
 use crate::sampler::{CompletedTrial, Sampler};
 
+use super::common::{from_internal, internal_bounds, sample_random};
+
 /// CMA-ES sampler for continuous optimization.
 ///
 /// Adapts a multivariate Gaussian to concentrate around promising regions
@@ -671,58 +673,6 @@ fn clip_to_bounds(x: &mut DVector<f64>, dimensions: &[DimensionInfo]) {
     }
 }
 
-/// Convert an internal-space value back to a `ParamValue`.
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn from_internal(value: f64, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low) / step).round();
-                d.low + k * step
-            } else {
-                v
-            };
-            ParamValue::Float(v.clamp(d.low, d.high))
-        }
-        Distribution::Int(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low as f64) / step as f64).round() as i64;
-                d.low + k * step
-            } else {
-                v.round() as i64
-            };
-            ParamValue::Int(v.clamp(d.low, d.high))
-        }
-        Distribution::Categorical(_) => {
-            unreachable!("from_internal should not be called for categorical distributions")
-        }
-    }
-}
-
-/// Compute internal-space bounds for a distribution.
-#[allow(clippy::cast_precision_loss)]
-fn internal_bounds(distribution: &Distribution) -> Option<(f64, f64)> {
-    match distribution {
-        Distribution::Float(d) => {
-            if d.log_scale {
-                Some((d.low.ln(), d.high.ln()))
-            } else {
-                Some((d.low, d.high))
-            }
-        }
-        Distribution::Int(d) => {
-            if d.log_scale {
-                Some(((d.low as f64).ln(), (d.high as f64).ln()))
-            } else {
-                Some((d.low as f64, d.high as f64))
-            }
-        }
-        Distribution::Categorical(_) => None,
-    }
-}
-
 /// Sample a value from the standard normal distribution using Box-Muller transform.
 fn sample_standard_normal(rng: &mut fastrand::Rng) -> f64 {
     // Box-Muller transform
@@ -731,51 +681,6 @@ fn sample_standard_normal(rng: &mut fastrand::Rng) -> f64 {
     (-2.0 * u1.ln()).sqrt() * u2.cos()
 }
 
-/// Sample a categorical value randomly.
-fn sample_random_categorical(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-        _ => unreachable!("sample_random_categorical called with non-categorical distribution"),
-    }
-}
-
-/// Sample a random value for any distribution (used during discovery phase).
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn sample_random(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let value = if d.log_scale {
-                let log_low = d.low.ln();
-                let log_high = d.high.ln();
-                rng_util::f64_range(rng, log_low, log_high).exp()
-            } else if let Some(step) = d.step {
-                let n_steps = ((d.high - d.low) / step).floor() as i64;
-                let k = rng.i64(0..=n_steps);
-                d.low + (k as f64) * step
-            } else {
-                rng_util::f64_range(rng, d.low, d.high)
-            };
-            ParamValue::Float(value)
-        }
-        Distribution::Int(d) => {
-            let value = if d.log_scale {
-                let log_low = (d.low as f64).ln();
-                let log_high = (d.high as f64).ln();
-                let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                raw.clamp(d.low, d.high)
-            } else if let Some(step) = d.step {
-                let n_steps = (d.high - d.low) / step;
-                let k = rng.i64(0..=n_steps);
-                d.low + k * step
-            } else {
-                rng.i64(d.low..=d.high)
-            };
-            ParamValue::Int(value)
-        }
-        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-    }
-}
-
 // ---------------------------------------------------------------------------
 // Sampler trait implementation
 // ---------------------------------------------------------------------------
@@ -920,7 +825,7 @@ fn sample_active(
         if let Some(&cat_idx) = candidate.categorical_values.get(&dim_idx) {
             ParamValue::Categorical(cat_idx)
         } else {
-            sample_random_categorical(&mut state.rng, distribution)
+            sample_random(&mut state.rng, distribution)
         }
     }
 }

src/sampler/common.rs

@@ -0,0 +1,116 @@
+//! Shared distribution-level utilities used across multiple samplers.
+
+use crate::distribution::Distribution;
+use crate::param::ParamValue;
+use crate::rng_util;
+
+/// Compute internal-space bounds for a distribution.
+#[allow(clippy::cast_precision_loss)]
+pub(crate) fn internal_bounds(distribution: &Distribution) -> Option<(f64, f64)> {
+    match distribution {
+        Distribution::Float(d) => {
+            if d.log_scale {
+                Some((d.low.ln(), d.high.ln()))
+            } else {
+                Some((d.low, d.high))
+            }
+        }
+        Distribution::Int(d) => {
+            if d.log_scale {
+                Some(((d.low as f64).ln(), (d.high as f64).ln()))
+            } else {
+                Some((d.low as f64, d.high as f64))
+            }
+        }
+        Distribution::Categorical(_) => None,
+    }
+}
+
+/// Convert an internal-space value back to a `ParamValue`.
+#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
+pub(crate) fn from_internal(value: f64, distribution: &Distribution) -> ParamValue {
+    match distribution {
+        Distribution::Float(d) => {
+            let v = if d.log_scale { value.exp() } else { value };
+            let v = if let Some(step) = d.step {
+                let k = ((v - d.low) / step).round();
+                d.low + k * step
+            } else {
+                v
+            };
+            ParamValue::Float(v.clamp(d.low, d.high))
+        }
+        Distribution::Int(d) => {
+            let v = if d.log_scale { value.exp() } else { value };
+            let v = if let Some(step) = d.step {
+                let k = ((v - d.low as f64) / step as f64).round() as i64;
+                d.low + k * step
+            } else {
+                v.round() as i64
+            };
+            ParamValue::Int(v.clamp(d.low, d.high))
+        }
+        Distribution::Categorical(_) => {
+            unreachable!("from_internal should not be called for categorical distributions")
+        }
+    }
+}
+
+/// Convert a `ParamValue` to its internal-space representation.
+#[allow(clippy::cast_precision_loss, dead_code)]
+pub(crate) fn to_internal(value: &ParamValue, distribution: &Distribution) -> f64 {
+    match (value, distribution) {
+        (ParamValue::Float(v), Distribution::Float(d)) => {
+            if d.log_scale {
+                v.ln()
+            } else {
+                *v
+            }
+        }
+        (ParamValue::Int(v), Distribution::Int(d)) => {
+            if d.log_scale {
+                (*v as f64).ln()
+            } else {
+                *v as f64
+            }
+        }
+        _ => 0.0,
+    }
+}
+
+/// Sample a random value for any distribution.
+#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
+pub(crate) fn sample_random(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
+    match distribution {
+        Distribution::Float(d) => {
+            let value = if d.log_scale {
+                let log_low = d.low.ln();
+                let log_high = d.high.ln();
+                rng_util::f64_range(rng, log_low, log_high).exp()
+            } else if let Some(step) = d.step {
+                let n_steps = ((d.high - d.low) / step).floor() as i64;
+                let k = rng.i64(0..=n_steps);
+                d.low + (k as f64) * step
+            } else {
+                rng_util::f64_range(rng, d.low, d.high)
+            };
+            ParamValue::Float(value)
+        }
+        Distribution::Int(d) => {
+            let value = if d.log_scale {
+                let log_low = (d.low as f64).ln();
+                let log_high = (d.high as f64).ln();
+                let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
+                raw.clamp(d.low, d.high)
+            } else if let Some(step) = d.step {
+                let n_steps = (d.high - d.low) / step;
+                let k = rng.i64(0..=n_steps);
+                d.low + k * step
+            } else {
+                rng.i64(d.low..=d.high)
+            };
+            ParamValue::Int(value)
+        }
+        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
+    }
+}

src/sampler/de.rs

@@ -70,6 +70,8 @@ use crate::param::ParamValue;
 use crate::rng_util;
 use crate::sampler::{CompletedTrial, Sampler};
 
+use super::common::{from_internal, internal_bounds, sample_random};
+
 /// Differential Evolution mutation strategy.
 ///
 /// Controls how mutant vectors are created from the current population.
@@ -396,95 +398,6 @@ impl State {
 // Helpers
 // ---------------------------------------------------------------------------
 
-/// Compute internal-space bounds for a distribution.
-#[allow(clippy::cast_precision_loss)]
-fn internal_bounds(distribution: &Distribution) -> Option<(f64, f64)> {
-    match distribution {
-        Distribution::Float(d) => {
-            if d.log_scale {
-                Some((d.low.ln(), d.high.ln()))
-            } else {
-                Some((d.low, d.high))
-            }
-        }
-        Distribution::Int(d) => {
-            if d.log_scale {
-                Some(((d.low as f64).ln(), (d.high as f64).ln()))
-            } else {
-                Some((d.low as f64, d.high as f64))
-            }
-        }
-        Distribution::Categorical(_) => None,
-    }
-}
-
-/// Convert an internal-space value back to a `ParamValue`.
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn from_internal(value: f64, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low) / step).round();
-                d.low + k * step
-            } else {
-                v
-            };
-            ParamValue::Float(v.clamp(d.low, d.high))
-        }
-        Distribution::Int(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low as f64) / step as f64).round() as i64;
-                d.low + k * step
-            } else {
-                v.round() as i64
-            };
-            ParamValue::Int(v.clamp(d.low, d.high))
-        }
-        Distribution::Categorical(_) => {
-            unreachable!("from_internal should not be called for categorical distributions")
-        }
-    }
-}
-
-/// Sample a random value for any distribution.
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn sample_random(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let value = if d.log_scale {
-                let log_low = d.low.ln();
-                let log_high = d.high.ln();
-                rng_util::f64_range(rng, log_low, log_high).exp()
-            } else if let Some(step) = d.step {
-                let n_steps = ((d.high - d.low) / step).floor() as i64;
-                let k = rng.i64(0..=n_steps);
-                d.low + (k as f64) * step
-            } else {
-                rng_util::f64_range(rng, d.low, d.high)
-            };
-            ParamValue::Float(value)
-        }
-        Distribution::Int(d) => {
-            let value = if d.log_scale {
-                let log_low = (d.low as f64).ln();
-                let log_high = (d.high as f64).ln();
-                let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                raw.clamp(d.low, d.high)
-            } else if let Some(step) = d.step {
-                let n_steps = (d.high - d.low) / step;
-                let k = rng.i64(0..=n_steps);
-                d.low + k * step
-            } else {
-                rng.i64(d.low..=d.high)
-            };
-            ParamValue::Int(value)
-        }
-        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-    }
-}
-
 /// Sample a random value in internal space for a continuous dimension.
 fn sample_random_internal(rng: &mut fastrand::Rng, bounds: (f64, f64)) -> f64 {
     rng_util::f64_range(rng, bounds.0, bounds.1)

src/sampler/genetic.rs

@@ -406,42 +406,7 @@ pub(crate) fn polynomial_mutation_f64(
     (x + delta_q * range).clamp(low, high)
 }
 
-/// Random sampling for a single distribution.
+pub(crate) use super::common::sample_random;
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-pub(crate) fn sample_random(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let value = if d.log_scale {
-                let log_low = d.low.ln();
-                let log_high = d.high.ln();
-                rng_util::f64_range(rng, log_low, log_high).exp()
-            } else if let Some(step) = d.step {
-                let n_steps = ((d.high - d.low) / step).floor() as i64;
-                let k = rng.i64(0..=n_steps);
-                d.low + (k as f64) * step
-            } else {
-                rng_util::f64_range(rng, d.low, d.high)
-            };
-            ParamValue::Float(value)
-        }
-        Distribution::Int(d) => {
-            let value = if d.log_scale {
-                let log_low = (d.low as f64).ln();
-                let log_high = (d.high as f64).ln();
-                let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                raw.clamp(d.low, d.high)
-            } else if let Some(step) = d.step {
-                let n_steps = (d.high - d.low) / step;
-                let k = rng.i64(0..=n_steps);
-                d.low + k * step
-            } else {
-                rng.i64(d.low..=d.high)
-            };
-            ParamValue::Int(value)
-        }
-        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-    }
-}
 
 // ---------------------------------------------------------------------------
 // Das-Dennis reference point generation

src/sampler/gp.rs

@@ -82,6 +82,8 @@ use crate::param::ParamValue;
 use crate::rng_util;
 use crate::sampler::{CompletedTrial, Sampler};
 
+use super::common::{from_internal, internal_bounds, sample_random, to_internal};
+
 // ---------------------------------------------------------------------------
 // Public API
 // ---------------------------------------------------------------------------
@@ -536,58 +538,6 @@ fn optimize_acquisition(
 // Data preprocessing helpers
 // ---------------------------------------------------------------------------
 
-/// Compute internal-space bounds for a distribution.
-#[allow(clippy::cast_precision_loss)]
-fn internal_bounds(distribution: &Distribution) -> Option<(f64, f64)> {
-    match distribution {
-        Distribution::Float(d) => {
-            if d.log_scale {
-                Some((d.low.ln(), d.high.ln()))
-            } else {
-                Some((d.low, d.high))
-            }
-        }
-        Distribution::Int(d) => {
-            if d.log_scale {
-                Some(((d.low as f64).ln(), (d.high as f64).ln()))
-            } else {
-                Some((d.low as f64, d.high as f64))
-            }
-        }
-        Distribution::Categorical(_) => None,
-    }
-}
-
-/// Convert a value from internal space to a `ParamValue` in original space.
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn from_internal(value: f64, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low) / step).round();
-                d.low + k * step
-            } else {
-                v
-            };
-            ParamValue::Float(v.clamp(d.low, d.high))
-        }
-        Distribution::Int(d) => {
-            let v = if d.log_scale { value.exp() } else { value };
-            let v = if let Some(step) = d.step {
-                let k = ((v - d.low as f64) / step as f64).round() as i64;
-                d.low + k * step
-            } else {
-                v.round() as i64
-            };
-            ParamValue::Int(v.clamp(d.low, d.high))
-        }
-        Distribution::Categorical(_) => {
-            unreachable!("from_internal should not be called for categorical distributions")
-        }
-    }
-}
-
 /// Convert an internal-space value to normalized [0, 1] using bounds.
 fn to_normalized(value: f64, lo: f64, hi: f64) -> f64 {
     if (hi - lo).abs() < 1e-15 {
@@ -602,65 +552,6 @@ fn from_normalized(value: f64, lo: f64, hi: f64) -> f64 {
     lo + value * (hi - lo)
 }
 
-/// Convert a `ParamValue` to its internal-space representation.
-#[allow(clippy::cast_precision_loss)]
-fn to_internal(value: &ParamValue, distribution: &Distribution) -> f64 {
-    match (value, distribution) {
-        (ParamValue::Float(v), Distribution::Float(d)) => {
-            if d.log_scale {
-                v.ln()
-            } else {
-                *v
-            }
-        }
-        (ParamValue::Int(v), Distribution::Int(d)) => {
-            if d.log_scale {
-                (*v as f64).ln()
-            } else {
-                *v as f64
-            }
-        }
-        _ => 0.0,
-    }
-}
-
-/// Sample a random value for any distribution.
-#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
-fn sample_random(rng: &mut fastrand::Rng, distribution: &Distribution) -> ParamValue {
-    match distribution {
-        Distribution::Float(d) => {
-            let value = if d.log_scale {
-                let log_low = d.low.ln();
-                let log_high = d.high.ln();
-                rng_util::f64_range(rng, log_low, log_high).exp()
-            } else if let Some(step) = d.step {
-                let n_steps = ((d.high - d.low) / step).floor() as i64;
-                let k = rng.i64(0..=n_steps);
-                d.low + (k as f64) * step
-            } else {
-                rng_util::f64_range(rng, d.low, d.high)
-            };
-            ParamValue::Float(value)
-        }
-        Distribution::Int(d) => {
-            let value = if d.log_scale {
-                let log_low = (d.low as f64).ln();
-                let log_high = (d.high as f64).ln();
-                let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                raw.clamp(d.low, d.high)
-            } else if let Some(step) = d.step {
-                let n_steps = (d.high - d.low) / step;
-                let k = rng.i64(0..=n_steps);
-                d.low + k * step
-            } else {
-                rng.i64(d.low..=d.high)
-            };
-            ParamValue::Int(value)
-        }
-        Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-    }
-}
-
 // ---------------------------------------------------------------------------
 // Extract training data from history
 // ---------------------------------------------------------------------------

src/sampler/mod.rs

@@ -3,6 +3,7 @@
 pub mod bohb;
 #[cfg(feature = "cma-es")]
 pub mod cma_es;
+pub(crate) mod common;
 pub mod de;
 pub(crate) mod genetic;
 #[cfg(feature = "gp")]

src/sampler/motpe.rs

@@ -61,9 +61,10 @@
 use core::sync::atomic::{AtomicU64, Ordering};
 
 use crate::distribution::Distribution;
-use crate::kde::KernelDensityEstimator;
 use crate::multi_objective::{MultiObjectiveSampler, MultiObjectiveTrial};
 use crate::param::ParamValue;
+use crate::sampler::common;
+use crate::sampler::tpe::common as tpe_common;
 use crate::types::{Direction, TrialState};
 use crate::{pareto, rng_util};
 
@@ -205,248 +206,6 @@ impl MotpeSampler {
 
         (good, bad)
     }
-
-    /// Samples uniformly from a distribution (used during startup phase).
-    #[allow(
-        clippy::cast_possible_truncation,
-        clippy::cast_precision_loss,
-        clippy::unused_self
-    )]
-    fn sample_uniform(distribution: &Distribution, rng: &mut fastrand::Rng) -> ParamValue {
-        match distribution {
-            Distribution::Float(d) => {
-                let value = if d.log_scale {
-                    let log_low = d.low.ln();
-                    let log_high = d.high.ln();
-                    rng_util::f64_range(rng, log_low, log_high).exp()
-                } else if let Some(step) = d.step {
-                    let n_steps = ((d.high - d.low) / step).floor() as i64;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + (k as f64) * step
-                } else {
-                    rng_util::f64_range(rng, d.low, d.high)
-                };
-                ParamValue::Float(value)
-            }
-            Distribution::Int(d) => {
-                let value = if d.log_scale {
-                    let log_low = (d.low as f64).ln();
-                    let log_high = (d.high as f64).ln();
-                    let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                    raw.clamp(d.low, d.high)
-                } else if let Some(step) = d.step {
-                    let n_steps = (d.high - d.low) / step;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + k * step
-                } else {
-                    rng.i64(d.low..=d.high)
-                };
-                ParamValue::Int(value)
-            }
-            Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-        }
-    }
-
-    /// Samples using TPE for float distributions.
-    #[allow(clippy::too_many_arguments)]
-    fn sample_tpe_float(
-        &self,
-        low: f64,
-        high: f64,
-        log_scale: bool,
-        step: Option<f64>,
-        good_values: Vec<f64>,
-        bad_values: Vec<f64>,
-        rng: &mut fastrand::Rng,
-    ) -> f64 {
-        // Transform to internal space (log space if needed)
-        let (internal_low, internal_high, good_internal, bad_internal) = if log_scale {
-            let i_low = low.ln();
-            let i_high = high.ln();
-            let g = {
-                let mut v = good_values;
-                for x in &mut v {
-                    *x = x.ln();
-                }
-                v
-            };
-            let b = {
-                let mut v = bad_values;
-                for x in &mut v {
-                    *x = x.ln();
-                }
-                v
-            };
-            (i_low, i_high, g, b)
-        } else {
-            (low, high, good_values, bad_values)
-        };
-
-        // Fit KDEs to good and bad groups
-        let l_kde = match self.kde_bandwidth {
-            Some(bw) => KernelDensityEstimator::with_bandwidth(good_internal, bw),
-            None => KernelDensityEstimator::new(good_internal),
-        };
-        let g_kde = match self.kde_bandwidth {
-            Some(bw) => KernelDensityEstimator::with_bandwidth(bad_internal, bw),
-            None => KernelDensityEstimator::new(bad_internal),
-        };
-
-        // If KDE construction fails, fall back to uniform sampling
-        let (Ok(l_kde), Ok(g_kde)) = (l_kde, g_kde) else {
-            return rng_util::f64_range(rng, low, high);
-        };
-
-        // Generate candidates from l(x) and select the one with best l(x)/g(x)
-        let mut best_candidate = internal_low;
-        let mut best_ratio = f64::NEG_INFINITY;
-
-        for _ in 0..self.n_ei_candidates {
-            let candidate = l_kde.sample(rng).clamp(internal_low, internal_high);
-
-            let l_density = l_kde.pdf(candidate);
-            let g_density = g_kde.pdf(candidate);
-
-            let ratio = if g_density < f64::EPSILON {
-                if l_density > f64::EPSILON {
-                    f64::INFINITY
-                } else {
-                    0.0
-                }
-            } else {
-                l_density / g_density
-            };
-
-            if ratio > best_ratio {
-                best_ratio = ratio;
-                best_candidate = candidate;
-            }
-        }
-
-        // Transform back from internal space
-        let mut value = if log_scale {
-            best_candidate.exp()
-        } else {
-            best_candidate
-        };
-
-        // Apply step constraint if present
-        if let Some(step) = step {
-            let k = ((value - low) / step).round();
-            value = low + k * step;
-        }
-
-        value.clamp(low, high)
-    }
-
-    /// Samples using TPE for integer distributions.
-    #[allow(
-        clippy::too_many_arguments,
-        clippy::cast_precision_loss,
-        clippy::cast_possible_truncation
-    )]
-    fn sample_tpe_int(
-        &self,
-        low: i64,
-        high: i64,
-        log_scale: bool,
-        step: Option<i64>,
-        good_values: Vec<i64>,
-        bad_values: Vec<i64>,
-        rng: &mut fastrand::Rng,
-    ) -> i64 {
-        let good_floats: Vec<f64> = good_values.into_iter().map(|v| v as f64).collect();
-        let bad_floats: Vec<f64> = bad_values.into_iter().map(|v| v as f64).collect();
-
-        let float_value = self.sample_tpe_float(
-            low as f64,
-            high as f64,
-            log_scale,
-            step.map(|s| s as f64),
-            good_floats,
-            bad_floats,
-            rng,
-        );
-
-        let int_value = float_value.round() as i64;
-        let int_value = if let Some(step) = step {
-            let k = ((int_value - low) as f64 / step as f64).round() as i64;
-            low + k * step
-        } else {
-            int_value
-        };
-
-        int_value.clamp(low, high)
-    }
-
-    /// Samples using TPE for categorical distributions.
-    #[allow(clippy::cast_precision_loss)]
-    fn sample_tpe_categorical(
-        n_choices: usize,
-        good_indices: &[usize],
-        bad_indices: &[usize],
-        rng: &mut fastrand::Rng,
-    ) -> usize {
-        // Stack-allocate for the common case (<=32 choices), heap for rare large cases
-        let mut good_buf = [0usize; 32];
-        let mut bad_buf = [0usize; 32];
-        let mut weight_buf = [0.0f64; 32];
-
-        let mut good_vec;
-        let mut bad_vec;
-        let mut weight_vec;
-
-        let (good_counts, bad_counts, weights): (&mut [usize], &mut [usize], &mut [f64]) =
-            if n_choices <= 32 {
-                (
-                    &mut good_buf[..n_choices],
-                    &mut bad_buf[..n_choices],
-                    &mut weight_buf[..n_choices],
-                )
-            } else {
-                good_vec = vec![0usize; n_choices];
-                bad_vec = vec![0usize; n_choices];
-                weight_vec = vec![0.0f64; n_choices];
-                (&mut good_vec, &mut bad_vec, &mut weight_vec)
-            };
-
-        // Count occurrences in good and bad groups
-        for &idx in good_indices {
-            if idx < n_choices {
-                good_counts[idx] += 1;
-            }
-        }
-        for &idx in bad_indices {
-            if idx < n_choices {
-                bad_counts[idx] += 1;
-            }
-        }
-
-        // Laplace smoothing
-        let good_total = good_indices.len() as f64 + n_choices as f64;
-        let bad_total = bad_indices.len() as f64 + n_choices as f64;
-
-        // Calculate l(x)/g(x) ratio for each category
-        for i in 0..n_choices {
-            let l_prob = (good_counts[i] as f64 + 1.0) / good_total;
-            let g_prob = (bad_counts[i] as f64 + 1.0) / bad_total;
-            weights[i] = l_prob / g_prob;
-        }
-
-        // Sample proportionally to weights
-        let total_weight: f64 = weights.iter().sum();
-        let threshold = rng.f64() * total_weight;
-
-        let mut cumulative = 0.0;
-        for (i, &w) in weights.iter().enumerate() {
-            cumulative += w;
-            if cumulative >= threshold {
-                return i;
-            }
-        }
-
-        n_choices - 1
-    }
 }
 
 impl Default for MotpeSampler {
@@ -477,14 +236,14 @@ impl MultiObjectiveSampler for MotpeSampler {
             .filter(|t| t.state == TrialState::Complete)
             .count();
         if n_complete < self.n_startup_trials {
-            return Self::sample_uniform(distribution, &mut rng);
+            return common::sample_random(&mut rng, distribution);
         }
 
         // Split trials into good (Pareto front) and bad (dominated)
         let (good_trials, bad_trials) = Self::split_trials(history, directions);
 
         if good_trials.is_empty() || bad_trials.is_empty() {
-            return Self::sample_uniform(distribution, &mut rng);
+            return common::sample_random(&mut rng, distribution);
         }
 
         match distribution {
@@ -510,16 +269,18 @@ impl MultiObjectiveSampler for MotpeSampler {
                     .collect();
 
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return Self::sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let value = self.sample_tpe_float(
+                let value = tpe_common::sample_tpe_float(
                     d.low,
                     d.high,
                     d.log_scale,
                     d.step,
                     good_values,
                     bad_values,
+                    self.n_ei_candidates,
+                    self.kde_bandwidth,
                     &mut rng,
                 );
                 ParamValue::Float(value)
@@ -546,16 +307,18 @@ impl MultiObjectiveSampler for MotpeSampler {
                     .collect();
 
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return Self::sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let value = self.sample_tpe_int(
+                let value = tpe_common::sample_tpe_int(
                     d.low,
                     d.high,
                     d.log_scale,
                     d.step,
                     good_values,
                     bad_values,
+                    self.n_ei_candidates,
+                    self.kde_bandwidth,
                     &mut rng,
                 );
                 ParamValue::Int(value)
@@ -582,10 +345,10 @@ impl MultiObjectiveSampler for MotpeSampler {
                     .collect();
 
                 if good_indices.is_empty() || bad_indices.is_empty() {
-                    return Self::sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let index = Self::sample_tpe_categorical(
+                let index = tpe_common::sample_tpe_categorical(
                     d.n_choices,
                     &good_indices,
                     &bad_indices,

src/sampler/random.rs

@@ -127,50 +127,7 @@ impl Sampler for RandomSampler {
             rng_util::distribution_fingerprint(distribution).wrapping_add(seq),
         ));
 
-        match distribution {
+        super::common::sample_random(&mut rng, distribution)
-            Distribution::Float(d) => {
-                let value = if d.log_scale {
-                    // Sample uniformly in log space
-                    let log_low = d.low.ln();
-                    let log_high = d.high.ln();
-                    let log_value = rng_util::f64_range(&mut rng, log_low, log_high);
-                    log_value.exp()
-                } else if let Some(step) = d.step {
-                    // Sample from step grid
-                    let n_steps = ((d.high - d.low) / step).floor() as i64;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + (k as f64) * step
-                } else {
-                    // Uniform sampling
-                    rng_util::f64_range(&mut rng, d.low, d.high)
-                };
-                ParamValue::Float(value)
-            }
-            Distribution::Int(d) => {
-                let value = if d.log_scale {
-                    // Sample uniformly in log space, then round
-                    let log_low = (d.low as f64).ln();
-                    let log_high = (d.high as f64).ln();
-                    let log_value = rng_util::f64_range(&mut rng, log_low, log_high);
-                    let raw = log_value.exp().round() as i64;
-                    // Clamp to bounds since rounding might push outside
-                    raw.clamp(d.low, d.high)
-                } else if let Some(step) = d.step {
-                    // Sample from step grid
-                    let n_steps = (d.high - d.low) / step;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + k * step
-                } else {
-                    // Uniform sampling
-                    rng.i64(d.low..=d.high)
-                };
-                ParamValue::Int(value)
-            }
-            Distribution::Categorical(d) => {
-                let index = rng.usize(0..d.n_choices);
-                ParamValue::Categorical(index)
-            }
-        }
     }
 }
 

src/sampler/tpe/common.rs

@@ -0,0 +1,218 @@
+//! Shared TPE sampling functions used by both `TpeSampler` and `MotpeSampler`.
+
+use crate::kde::KernelDensityEstimator;
+use crate::rng_util;
+
+/// Samples using TPE for float distributions.
+#[allow(clippy::too_many_arguments)]
+pub(crate) fn sample_tpe_float(
+    low: f64,
+    high: f64,
+    log_scale: bool,
+    step: Option<f64>,
+    good_values: Vec<f64>,
+    bad_values: Vec<f64>,
+    n_ei_candidates: usize,
+    kde_bandwidth: Option<f64>,
+    rng: &mut fastrand::Rng,
+) -> f64 {
+    // Transform to internal space (log space if needed)
+    let (internal_low, internal_high, good_internal, bad_internal) = if log_scale {
+        let i_low = low.ln();
+        let i_high = high.ln();
+        let g = {
+            let mut v = good_values;
+            for x in &mut v {
+                *x = x.ln();
+            }
+            v
+        };
+        let b = {
+            let mut v = bad_values;
+            for x in &mut v {
+                *x = x.ln();
+            }
+            v
+        };
+        (i_low, i_high, g, b)
+    } else {
+        (low, high, good_values, bad_values)
+    };
+
+    // Fit KDEs to good and bad groups
+    let l_kde = match kde_bandwidth {
+        Some(bw) => KernelDensityEstimator::with_bandwidth(good_internal, bw),
+        None => KernelDensityEstimator::new(good_internal),
+    };
+    let g_kde = match kde_bandwidth {
+        Some(bw) => KernelDensityEstimator::with_bandwidth(bad_internal, bw),
+        None => KernelDensityEstimator::new(bad_internal),
+    };
+
+    // If KDE construction fails, fall back to uniform sampling
+    let (Ok(l_kde), Ok(g_kde)) = (l_kde, g_kde) else {
+        return rng_util::f64_range(rng, low, high);
+    };
+
+    // Generate candidates from l(x) and select the one with best l(x)/g(x) ratio
+    let mut best_candidate = internal_low;
+    let mut best_ratio = f64::NEG_INFINITY;
+
+    for _ in 0..n_ei_candidates {
+        let candidate = l_kde.sample(rng).clamp(internal_low, internal_high);
+
+        let l_density = l_kde.pdf(candidate);
+        let g_density = g_kde.pdf(candidate);
+
+        // Compute l(x)/g(x) ratio, handling zero density
+        let ratio = if g_density < f64::EPSILON {
+            if l_density > f64::EPSILON {
+                f64::INFINITY
+            } else {
+                0.0
+            }
+        } else {
+            l_density / g_density
+        };
+
+        if ratio > best_ratio {
+            best_ratio = ratio;
+            best_candidate = candidate;
+        }
+    }
+
+    // Transform back from internal space
+    let mut value = if log_scale {
+        best_candidate.exp()
+    } else {
+        best_candidate
+    };
+
+    // Apply step constraint if present
+    if let Some(step) = step {
+        let k = ((value - low) / step).round();
+        value = low + k * step;
+    }
+
+    // Ensure value is within bounds
+    value.clamp(low, high)
+}
+
+/// Samples using TPE for integer distributions.
+#[allow(
+    clippy::too_many_arguments,
+    clippy::cast_precision_loss,
+    clippy::cast_possible_truncation
+)]
+pub(crate) fn sample_tpe_int(
+    low: i64,
+    high: i64,
+    log_scale: bool,
+    step: Option<i64>,
+    good_values: Vec<i64>,
+    bad_values: Vec<i64>,
+    n_ei_candidates: usize,
+    kde_bandwidth: Option<f64>,
+    rng: &mut fastrand::Rng,
+) -> i64 {
+    // Convert to floats for KDE
+    let good_floats: Vec<f64> = good_values.into_iter().map(|v| v as f64).collect();
+    let bad_floats: Vec<f64> = bad_values.into_iter().map(|v| v as f64).collect();
+
+    // Use float TPE sampling
+    let float_value = sample_tpe_float(
+        low as f64,
+        high as f64,
+        log_scale,
+        step.map(|s| s as f64),
+        good_floats,
+        bad_floats,
+        n_ei_candidates,
+        kde_bandwidth,
+        rng,
+    );
+
+    // Round to nearest integer
+    let int_value = float_value.round() as i64;
+
+    // Apply step constraint if present
+    let int_value = if let Some(step) = step {
+        let k = ((int_value - low) as f64 / step as f64).round() as i64;
+        low + k * step
+    } else {
+        int_value
+    };
+
+    // Ensure value is within bounds
+    int_value.clamp(low, high)
+}
+
+/// Samples using TPE for categorical distributions.
+#[allow(clippy::cast_precision_loss)]
+pub(crate) fn sample_tpe_categorical(
+    n_choices: usize,
+    good_indices: &[usize],
+    bad_indices: &[usize],
+    rng: &mut fastrand::Rng,
+) -> usize {
+    // Stack-allocate for the common case (<=32 choices), heap for rare large cases
+    let mut good_buf = [0usize; 32];
+    let mut bad_buf = [0usize; 32];
+    let mut weight_buf = [0.0f64; 32];
+
+    let mut good_vec;
+    let mut bad_vec;
+    let mut weight_vec;
+
+    let (good_counts, bad_counts, weights): (&mut [usize], &mut [usize], &mut [f64]) =
+        if n_choices <= 32 {
+            (
+                &mut good_buf[..n_choices],
+                &mut bad_buf[..n_choices],
+                &mut weight_buf[..n_choices],
+            )
+        } else {
+            good_vec = vec![0usize; n_choices];
+            bad_vec = vec![0usize; n_choices];
+            weight_vec = vec![0.0f64; n_choices];
+            (&mut good_vec, &mut bad_vec, &mut weight_vec)
+        };
+
+    // Count occurrences in good and bad groups
+    for &idx in good_indices {
+        if idx < n_choices {
+            good_counts[idx] += 1;
+        }
+    }
+    for &idx in bad_indices {
+        if idx < n_choices {
+            bad_counts[idx] += 1;
+        }
+    }
+
+    // Add smoothing (Laplace smoothing) to avoid zero probabilities
+    let good_total = good_indices.len() as f64 + n_choices as f64;
+    let bad_total = bad_indices.len() as f64 + n_choices as f64;
+
+    // Calculate l(x)/g(x) ratio for each category
+    for i in 0..n_choices {
+        let l_prob = (good_counts[i] as f64 + 1.0) / good_total;
+        let g_prob = (bad_counts[i] as f64 + 1.0) / bad_total;
+        weights[i] = l_prob / g_prob;
+    }
+
+    // Sample proportionally to weights
+    let total_weight: f64 = weights.iter().sum();
+    let threshold = rng.f64() * total_weight;
+
+    let mut cumulative = 0.0;
+    for (i, &w) in weights.iter().enumerate() {
+        cumulative += w;
+        if cumulative >= threshold {
+            return i;
+        }
+    }
+
+    // Fallback to last index (shouldn't happen)
+    n_choices - 1
+}

src/sampler/tpe/mod.rs

@@ -59,6 +59,7 @@
 //! let study: Study<f64> = Study::with_sampler(Direction::Minimize, sampler);
 //! ```
 
+pub(crate) mod common;
 mod gamma;
 mod multivariate;
 mod sampler;

src/sampler/tpe/multivariate.rs

@@ -810,55 +810,10 @@ impl MultivariateTpeSampler {
     ) -> HashMap<ParamId, ParamValue> {
         search_space
             .iter()
-            .map(|(id, dist)| (*id, Self::sample_uniform_single(dist, rng)))
+            .map(|(id, dist)| (*id, crate::sampler::common::sample_random(rng, dist)))
             .collect()
     }
 
-    /// Samples a single parameter uniformly at random from its distribution.
-    fn sample_uniform_single(distribution: &Distribution, rng: &mut fastrand::Rng) -> ParamValue {
-        match distribution {
-            Distribution::Float(d) => {
-                let value = if d.log_scale {
-                    let log_low = d.low.ln();
-                    let log_high = d.high.ln();
-                    rng_util::f64_range(rng, log_low, log_high).exp()
-                } else if let Some(step) = d.step {
-                    #[allow(clippy::cast_possible_truncation)]
-                    let n_steps = ((d.high - d.low) / step).floor() as i64;
-                    let k = rng.i64(0..=n_steps);
-                    #[allow(clippy::cast_precision_loss)]
-                    let result = d.low + (k as f64) * step;
-                    result
-                } else {
-                    rng_util::f64_range(rng, d.low, d.high)
-                };
-                ParamValue::Float(value)
-            }
-            Distribution::Int(d) => {
-                #[allow(clippy::cast_precision_loss)]
-                let value = if d.log_scale {
-                    let log_low = (d.low as f64).ln();
-                    let log_high = (d.high as f64).ln();
-                    #[allow(clippy::cast_possible_truncation)]
-                    let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                    raw.clamp(d.low, d.high)
-                } else if let Some(step) = d.step {
-                    #[allow(clippy::cast_possible_truncation)]
-                    let n_steps = (d.high - d.low) / step;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + k * step
-                } else {
-                    rng.i64(d.low..=d.high)
-                };
-                ParamValue::Int(value)
-            }
-            Distribution::Categorical(d) => {
-                let index = rng.usize(0..d.n_choices);
-                ParamValue::Categorical(index)
-            }
-        }
-    }
-
     /// Samples parameters jointly using multivariate TPE.
     ///
     /// This method samples all parameters in the search space jointly, capturing
@@ -1025,7 +980,7 @@ impl MultivariateTpeSampler {
             // Sample ungrouped parameters uniformly (no history for them)
             let mut rng = self.rng.lock();
             for (id, dist) in &ungrouped_params {
-                let value = Self::sample_uniform_single(dist, &mut rng);
+                let value = crate::sampler::common::sample_random(&mut rng, dist);
                 result.insert(*id, value);
             }
         }
@@ -1312,7 +1267,7 @@ impl MultivariateTpeSampler {
                     .collect();
 
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return Self::sample_uniform_single(distribution, rng);
+                    return crate::sampler::common::sample_random(rng, distribution);
                 }
 
                 let value = self.sample_tpe_float(
@@ -1348,7 +1303,7 @@ impl MultivariateTpeSampler {
                     .collect();
 
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return Self::sample_uniform_single(distribution, rng);
+                    return crate::sampler::common::sample_random(rng, distribution);
                 }
 
                 let value = self.sample_tpe_int(
@@ -1384,7 +1339,7 @@ impl MultivariateTpeSampler {
                     .collect();
 
                 if good_indices.is_empty() || bad_indices.is_empty() {
-                    return Self::sample_uniform_single(distribution, rng);
+                    return crate::sampler::common::sample_random(rng, distribution);
                 }
 
                 let idx =
@@ -1765,7 +1720,7 @@ impl Sampler for MultivariateTpeSampler {
         Self::find_matching_param(distribution, &joint_sample).unwrap_or_else(|| {
             // Fallback to uniform sampling if no match found
             let mut rng = self.rng.lock();
-            Self::sample_uniform_single(distribution, &mut rng)
+            crate::sampler::common::sample_random(&mut rng, distribution)
         })
     }
 }

src/sampler/tpe/sampler.rs

@@ -61,12 +61,14 @@ use std::sync::Arc;
 
 use crate::distribution::Distribution;
 use crate::error::{Error, Result};
-use crate::kde::KernelDensityEstimator;
 use crate::param::ParamValue;
 use crate::rng_util;
+use crate::sampler::common;
 use crate::sampler::tpe::gamma::{FixedGamma, GammaStrategy};
 use crate::sampler::{CompletedTrial, Sampler};
 
+use super::common as tpe_common;
+
 // ============================================================================
 // Gamma Strategy Trait and Implementations
 // ============================================================================
@@ -315,261 +317,6 @@ impl TpeSampler {
 
         (good, bad)
     }
-
-    /// Samples uniformly from a distribution (used during startup phase).
-    #[allow(
-        clippy::cast_possible_truncation,
-        clippy::cast_precision_loss,
-        clippy::unused_self
-    )]
-    fn sample_uniform(&self, distribution: &Distribution, rng: &mut fastrand::Rng) -> ParamValue {
-        match distribution {
-            Distribution::Float(d) => {
-                let value = if d.log_scale {
-                    let log_low = d.low.ln();
-                    let log_high = d.high.ln();
-                    rng_util::f64_range(rng, log_low, log_high).exp()
-                } else if let Some(step) = d.step {
-                    let n_steps = ((d.high - d.low) / step).floor() as i64;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + (k as f64) * step
-                } else {
-                    rng_util::f64_range(rng, d.low, d.high)
-                };
-                ParamValue::Float(value)
-            }
-            Distribution::Int(d) => {
-                let value = if d.log_scale {
-                    let log_low = (d.low as f64).ln();
-                    let log_high = (d.high as f64).ln();
-                    let raw = rng_util::f64_range(rng, log_low, log_high).exp().round() as i64;
-                    raw.clamp(d.low, d.high)
-                } else if let Some(step) = d.step {
-                    let n_steps = (d.high - d.low) / step;
-                    let k = rng.i64(0..=n_steps);
-                    d.low + k * step
-                } else {
-                    rng.i64(d.low..=d.high)
-                };
-                ParamValue::Int(value)
-            }
-            Distribution::Categorical(d) => ParamValue::Categorical(rng.usize(0..d.n_choices)),
-        }
-    }
-
-    /// Samples using TPE for float distributions.
-    #[allow(clippy::too_many_arguments)]
-    fn sample_tpe_float(
-        &self,
-        low: f64,
-        high: f64,
-        log_scale: bool,
-        step: Option<f64>,
-        good_values: Vec<f64>,
-        bad_values: Vec<f64>,
-        rng: &mut fastrand::Rng,
-    ) -> f64 {
-        // Transform to internal space (log space if needed)
-        let (internal_low, internal_high, good_internal, bad_internal) = if log_scale {
-            let i_low = low.ln();
-            let i_high = high.ln();
-            let g = {
-                let mut v = good_values;
-                for x in &mut v {
-                    *x = x.ln();
-                }
-                v
-            };
-            let b = {
-                let mut v = bad_values;
-                for x in &mut v {
-                    *x = x.ln();
-                }
-                v
-            };
-            (i_low, i_high, g, b)
-        } else {
-            (low, high, good_values, bad_values)
-        };
-
-        // Fit KDEs to good and bad groups
-        let l_kde = match self.kde_bandwidth {
-            Some(bw) => KernelDensityEstimator::with_bandwidth(good_internal, bw),
-            None => KernelDensityEstimator::new(good_internal),
-        };
-        let g_kde = match self.kde_bandwidth {
-            Some(bw) => KernelDensityEstimator::with_bandwidth(bad_internal, bw),
-            None => KernelDensityEstimator::new(bad_internal),
-        };
-
-        // If KDE construction fails, fall back to uniform sampling
-        let (Ok(l_kde), Ok(g_kde)) = (l_kde, g_kde) else {
-            return rng_util::f64_range(rng, low, high);
-        };
-
-        // Generate candidates from l(x) and select the one with best l(x)/g(x) ratio
-        let mut best_candidate = internal_low;
-        let mut best_ratio = f64::NEG_INFINITY;
-
-        for _ in 0..self.n_ei_candidates {
-            let candidate = l_kde.sample(rng);
-
-            // Clamp to bounds
-            let candidate = candidate.clamp(internal_low, internal_high);
-
-            let l_density = l_kde.pdf(candidate);
-            let g_density = g_kde.pdf(candidate);
-
-            // Compute l(x)/g(x) ratio, handling zero density
-            let ratio = if g_density < f64::EPSILON {
-                if l_density > f64::EPSILON {
-                    f64::INFINITY
-                } else {
-                    0.0
-                }
-            } else {
-                l_density / g_density
-            };
-
-            if ratio > best_ratio {
-                best_ratio = ratio;
-                best_candidate = candidate;
-            }
-        }
-
-        // Transform back from internal space
-        let mut value = if log_scale {
-            best_candidate.exp()
-        } else {
-            best_candidate
-        };
-
-        // Apply step constraint if present
-        if let Some(step) = step {
-            let k = ((value - low) / step).round();
-            value = low + k * step;
-        }
-
-        // Ensure value is within bounds
-        value.clamp(low, high)
-    }
-
-    /// Samples using TPE for integer distributions.
-    #[allow(
-        clippy::too_many_arguments,
-        clippy::cast_precision_loss,
-        clippy::cast_possible_truncation
-    )]
-    fn sample_tpe_int(
-        &self,
-        low: i64,
-        high: i64,
-        log_scale: bool,
-        step: Option<i64>,
-        good_values: Vec<i64>,
-        bad_values: Vec<i64>,
-        rng: &mut fastrand::Rng,
-    ) -> i64 {
-        // Convert to floats for KDE
-        let good_floats: Vec<f64> = good_values.into_iter().map(|v| v as f64).collect();
-        let bad_floats: Vec<f64> = bad_values.into_iter().map(|v| v as f64).collect();
-
-        // Use float TPE sampling
-        let float_value = self.sample_tpe_float(
-            low as f64,
-            high as f64,
-            log_scale,
-            step.map(|s| s as f64),
-            good_floats,
-            bad_floats,
-            rng,
-        );
-
-        // Round to nearest integer
-        let int_value = float_value.round() as i64;
-
-        // Apply step constraint if present
-        let int_value = if let Some(step) = step {
-            let k = ((int_value - low) as f64 / step as f64).round() as i64;
-            low + k * step
-        } else {
-            int_value
-        };
-
-        // Ensure value is within bounds
-        int_value.clamp(low, high)
-    }
-
-    /// Samples using TPE for categorical distributions.
-    #[allow(clippy::cast_precision_loss, clippy::unused_self)]
-    fn sample_tpe_categorical(
-        &self,
-        n_choices: usize,
-        good_indices: &[usize],
-        bad_indices: &[usize],
-        rng: &mut fastrand::Rng,
-    ) -> usize {
-        // Stack-allocate for the common case (<=32 choices), heap for rare large cases
-        let mut good_buf = [0usize; 32];
-        let mut bad_buf = [0usize; 32];
-        let mut weight_buf = [0.0f64; 32];
-
-        let mut good_vec;
-        let mut bad_vec;
-        let mut weight_vec;
-
-        let (good_counts, bad_counts, weights): (&mut [usize], &mut [usize], &mut [f64]) =
-            if n_choices <= 32 {
-                (
-                    &mut good_buf[..n_choices],
-                    &mut bad_buf[..n_choices],
-                    &mut weight_buf[..n_choices],
-                )
-            } else {
-                good_vec = vec![0usize; n_choices];
-                bad_vec = vec![0usize; n_choices];
-                weight_vec = vec![0.0f64; n_choices];
-                (&mut good_vec, &mut bad_vec, &mut weight_vec)
-            };
-
-        // Count occurrences in good and bad groups
-        for &idx in good_indices {
-            if idx < n_choices {
-                good_counts[idx] += 1;
-            }
-        }
-        for &idx in bad_indices {
-            if idx < n_choices {
-                bad_counts[idx] += 1;
-            }
-        }
-
-        // Add smoothing (Laplace smoothing) to avoid zero probabilities
-        let good_total = good_indices.len() as f64 + n_choices as f64;
-        let bad_total = bad_indices.len() as f64 + n_choices as f64;
-
-        // Calculate l(x)/g(x) ratio for each category
-        for i in 0..n_choices {
-            let l_prob = (good_counts[i] as f64 + 1.0) / good_total;
-            let g_prob = (bad_counts[i] as f64 + 1.0) / bad_total;
-            weights[i] = l_prob / g_prob;
-        }
-
-        // Sample proportionally to weights
-        let total_weight: f64 = weights.iter().sum();
-        let threshold = rng.f64() * total_weight;
-
-        let mut cumulative = 0.0;
-        for (i, &w) in weights.iter().enumerate() {
-            cumulative += w;
-            if cumulative >= threshold {
-                return i;
-            }
-        }
-
-        // Fallback to last index (shouldn't happen)
-        n_choices - 1
-    }
 }
 
 impl Default for TpeSampler {
@@ -912,7 +659,7 @@ impl Sampler for TpeSampler {
 
         // Fall back to random sampling during startup phase
         if history.len() < self.n_startup_trials {
-            return self.sample_uniform(distribution, &mut rng);
+            return common::sample_random(&mut rng, distribution);
         }
 
         // Split trials into good and bad groups
@@ -920,7 +667,7 @@ impl Sampler for TpeSampler {
 
         // Need at least 1 trial in each group for TPE
         if good_trials.is_empty() || bad_trials.is_empty() {
-            return self.sample_uniform(distribution, &mut rng);
+            return common::sample_random(&mut rng, distribution);
         }
 
         // Extract parameter values for this distribution
@@ -954,16 +701,18 @@ impl Sampler for TpeSampler {
 
                 // Need values in both groups for TPE
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return self.sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let value = self.sample_tpe_float(
+                let value = tpe_common::sample_tpe_float(
                     d.low,
                     d.high,
                     d.log_scale,
                     d.step,
                     good_values,
                     bad_values,
+                    self.n_ei_candidates,
+                    self.kde_bandwidth,
                     &mut rng,
                 );
                 ParamValue::Float(value)
@@ -990,16 +739,18 @@ impl Sampler for TpeSampler {
                     .collect();
 
                 if good_values.is_empty() || bad_values.is_empty() {
-                    return self.sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let value = self.sample_tpe_int(
+                let value = tpe_common::sample_tpe_int(
                     d.low,
                     d.high,
                     d.log_scale,
                     d.step,
                     good_values,
                     bad_values,
+                    self.n_ei_candidates,
+                    self.kde_bandwidth,
                     &mut rng,
                 );
                 ParamValue::Int(value)
@@ -1026,11 +777,15 @@ impl Sampler for TpeSampler {
                     .collect();
 
                 if good_indices.is_empty() || bad_indices.is_empty() {
-                    return self.sample_uniform(distribution, &mut rng);
+                    return common::sample_random(&mut rng, distribution);
                 }
 
-                let index =
+                let index = tpe_common::sample_tpe_categorical(
-                    self.sample_tpe_categorical(d.n_choices, &good_indices, &bad_indices, &mut rng);
+                    d.n_choices,
+                    &good_indices,
+                    &bad_indices,
+                    &mut rng,
+                );
                 ParamValue::Categorical(index)
             }
         }