feat(capability): 转码能力包（vendor 纯 Go AMR-NB 解码器 + 黄金向量逐样本对照 + 8 哨兵 + 注册面）

internal/capability/transcode/amrnb/dsp_dec_gain.go

@@ -0,0 +1,101 @@
+package amrnb
+
+// Gain dequantization, ported from opencore-amrnb d_gain_p.cpp, d_gain_c.cpp and
+// dec_gain.cpp. These turn the received gain indices into the per-subframe pitch
+// gain and fixed-codebook gain, driving the gc_pred energy predictor. dGainPitch
+// and dGainCode handle MR795/MR122; decGain handles the joint/scalar-VQ modes
+// (MR475 joint, MR515/MR59 low-rate, MR67/MR74/MR102 high-rate). Bit-exact with
+// the reference.
+
+const cMR475_VQ_SIZE = 256
+
+// dGainPitch returns the quantized pitch gain (Q14) for the given index.
+// Mirrors d_gain_pitch.
+func dGainPitch(mode int, index int16) int16 {
+	gain := qua_gain_pitch[index]
+	if mode == dMR122 {
+		gain &^= 3 // clear 2 LSBits
+	}
+	return gain
+}
+
+// dGainCode decodes the fixed-codebook gain for MR795/MR122 from the predicted
+// gain (gc_pred) and the quantized correction factor, then updates the
+// predictor. Mirrors d_gain_code.
+func (pred *gcPredState) dGainCode(mode int, index int16, code []int16) (gainCode int16) {
+	r := pred.gcPred(mode, code)
+	exp, frac := r.expGcode0, r.fracGcode0
+
+	index &= 31 // index < 32, avoid buffer overflow
+	tblTmp := index + (index << 1)
+	p := qua_gain_code[tblTmp:]
+
+	if mode == dMR122 {
+		gcode0 := int16(pow2(exp, frac))
+		gcode0 = shl(gcode0, 4)
+		gainCode = shl(mult(gcode0, p[0]), 1)
+	} else {
+		gcode0 := int16(pow2(14, frac))
+		lTmp := L_mult(p[0], gcode0)
+		lTmp = L_shr(lTmp, sub(9, exp))
+		gainCode = int16(lTmp >> 16) // Q1
+	}
+
+	quaEnerMR122 := p[1]
+	quaEner := p[2]
+	pred.update(quaEnerMR122, quaEner)
+	return gainCode
+}
+
+// decGain decodes both the pitch gain and fixed-codebook gain for the
+// scalar/joint-VQ modes (all except MR795/MR122), updating the predictor.
+// evenSubfr selects the MR475 joint-gain half. Mirrors Dec_gain.
+func (pred *gcPredState) decGain(mode int, index int16, code []int16, evenSubfr int16) (gainPit, gainCod int16) {
+	index = shl(index, 2)
+
+	var gCode, quaEnerMR122, quaEner int16
+
+	switch {
+	case mode == dMR102 || mode == dMR74 || mode == dMR67:
+		p := table_gain_highrates[index:]
+		gainPit = p[0]
+		gCode = p[1]
+		quaEnerMR122 = p[2]
+		quaEner = p[3]
+	case mode == dMR475:
+		index += (1 ^ evenSubfr) << 1 // evenSubfr is 0 or 1
+		if index > int16(cMR475_VQ_SIZE*4-2) {
+			index = int16(cMR475_VQ_SIZE*4 - 2)
+		}
+		p := table_gain_MR475[index:]
+		gainPit = p[0]
+		gCode = p[1]
+		// Predictor update values are recomputed (not stored in the 4.75 table):
+		//   qua_ener_MR122 = log2(g),  qua_ener = 20*log10(g)
+		e, f := log2(int32(gCode)) // Log2(x Q12) = log2(x) + 12
+		e -= 12
+		t1 := shr_r(f, 5)
+		t2 := shl(e, 10)
+		quaEnerMR122 = add(t1, t2)
+		lTmp := mpy32_16(e, f, 24660) // 24660 Q12 ~= 20*log10(2)
+		lTmp = L_shl(lTmp, 13)
+		quaEner = round_(lTmp)
+	default: // MR515, MR59
+		p := table_gain_lowrates[index:]
+		gainPit = p[0]
+		gCode = p[1]
+		quaEnerMR122 = p[2]
+		quaEner = p[3]
+	}
+
+	r := pred.gcPred(mode, code)
+	exp, frac := r.expGcode0, r.fracGcode0
+	gcode0 := int16(pow2(14, frac))
+
+	lTmp := L_mult(gCode, gcode0)
+	lTmp = L_shr(lTmp, 10-exp)
+	gainCod = int16(lTmp >> 16)
+
+	pred.update(quaEnerMR122, quaEner)
+	return gainPit, gainCod
+}