// Licensed under the Apache License, Version 2.0
// © mihakralj
//@version=6
// Fisher04: Ehlers Fisher Transform (2004 Cybernetic Analysis)
// Source: John Ehlers, "Cybernetic Analysis for Stocks and Futures", Wiley, 2004, Chapter 1
//
// Key differences from 2002 TASC article (fisher.pine):
//   Normalization: 1.0 * ((price-low)/range - 0.5) vs 0.66 * (...)
//   IIR feedback on Value1: 0.5 vs 0.67
//   Clamp threshold: 0.9999 vs 0.99→0.999
//   Fisher multiplier: 0.25 vs 0.5
//   Fisher IIR: 0.5 (same)
indicator("Ehlers Fisher Transform 2004 (FISHER04)", "FISHER04", overlay=false)

length = input.int(10, "Length", minval=1)

price = hl2

maxH = ta.highest(price, length)
minL = ta.lowest(price, length)

var float value1 = 0.0
var float fisher = 0.0
var float signal = 0.0

range_ = maxH - minL

if range_ != 0
    // Ehlers 2004: normalization coefficient = 1.0 (0.5 * 2)
    value1 := ((price - minL) / range_ - 0.5) + 0.5 * nz(value1[1])
else
    value1 := 0.0

// Ehlers 2004: clamp to ±0.9999
value1 := math.max(math.min(value1, 0.9999), -0.9999)

// Ehlers 2004: 0.25 * arctanh + 0.5 * Fish[1]
signal := fisher
fisher := 0.25 * math.log((1 + value1) / (1 - value1)) + 0.5 * nz(fisher[1])

plot(fisher, "Fisher04", color.yellow, 2)
plot(signal, "Signal", color.orange, 1)
hline(0, "Zero", color.gray, linestyle=hline.style_dotted)