package synth

import (
	"encoding/binary"
	"math"
)

// PanGains returns left and right channel gains for a pan position p in [-1, 1].
// Uses constant-power (equal-power) pan law: cos/sin mapping.
// Per D-11: bass frequencies center, mid spread L/R, higher frequencies wider.
// At p=-1.0: gainL=1.0, gainR=0.0; at p=0.0: gainL=gainR=sqrt(2)/2; at p=1.0: gainL=0.0, gainR=1.0.
func PanGains(p float64) (gainL, gainR float64) {
	angle := (p + 1.0) / 2.0 * math.Pi / 2.0
	return math.Cos(angle), math.Sin(angle)
}

// StereoFramesToInt16Bytes converts [][2]float64 stereo frames to interleaved
// little-endian int16 bytes suitable for go-lame's Write method.
// Clamps values to [-1.0, 1.0] before conversion.
// Output format: [L0_lo, L0_hi, R0_lo, R0_hi, L1_lo, L1_hi, R1_lo, R1_hi, ...]
func StereoFramesToInt16Bytes(frames [][2]float64) []byte {
	buf := make([]byte, len(frames)*4) // 2 channels * 2 bytes per sample
	for i, frame := range frames {
		l := clamp(frame[0])
		r := clamp(frame[1])
		binary.LittleEndian.PutUint16(buf[i*4:], uint16(int16(l*32767)))
		binary.LittleEndian.PutUint16(buf[i*4+2:], uint16(int16(r*32767)))
	}
	return buf
}

func clamp(v float64) float64 {
	if v > 1.0 {
		return 1.0
	}
	if v < -1.0 {
		return -1.0
	}
	return v
}