/* reads 16-bit stereo PCM on stdin, writes sped up mono mp3 on stdout */
/* adapted from https://github.com/waywardgeek/sonic */
#include <u.h>
#include <libc.h>
enum{MAXPERIOD = 672, MINPERIOD = 108, SKIP = 12, INSAMPS = 2048};
s16int input[2 * MAXPERIOD + INSAMPS];
s16int output[2 * MAXPERIOD];
s16int down[2 * MAXPERIOD / SKIP];
float speed = 2.0;
int nin, nout, nremain, minΔ, maxΔ, prev, prevminΔ;
void cp(s16int *to, s16int *from, int n){
	while(n--)
		*to++ = *from++;
}
int findperiodlim(s16int *in, int min, int max){
	int period, best = 0, worst = 255, Δ, i;
	s16int *s, *p, sval, pval;
	minΔ = 1;
	maxΔ = 0;
	for(period = min; period <= max; period++){
		s = in;
		p = in + period;
		for(Δ = i = 0; i < period; i++){
			sval = *s++;
			pval = *p++;
			Δ += sval >= pval ? sval - pval : pval - sval;
		}
		if(Δ * best < minΔ * period){
			minΔ = Δ;
			best = period;
		}
		if(Δ * worst > maxΔ * period){
			maxΔ = Δ;
			worst = period;
		}
	}
	minΔ /= best;
	maxΔ /= worst;
	return best;
}
void downsample(s16int *in, s16int *out){
	int i, j, val;
	for(i = 2 * MAXPERIOD / SKIP; i; i--){
		for(val = j = 0; j < SKIP; j++) val += *in++;
		*out++ = val / SKIP;
	}
}
int findperiod(s16int *in){
	int min, max, period;
	downsample(in, down);
	period = findperiodlim(down, MINPERIOD / SKIP, MAXPERIOD / SKIP) * SKIP;
	min = period - (SKIP << 2);
	if(min < MINPERIOD) min = MINPERIOD;
	max = period + (SKIP << 2);
	if(max > MAXPERIOD) max = MAXPERIOD;
	period = findperiodlim(in, min, max);
	/* at abrupt ends of words the previous period can be a better estimate  */
	max = minΔ == 0 || prev == 0 || maxΔ > minΔ * 3 || minΔ * 2 <= prevminΔ * 3 ? period : prev;
	prevminΔ = minΔ;
	prev = period;
	return max;
}
void resample(void){
	s16int *in, *out, *↓, *↑;
	int pos = 0, period, n, i;
	if(nin < 2 * MAXPERIOD) return;
	do{
		if(nremain > 0){
			n = nremain < 2 * MAXPERIOD ? nremain : 2 * MAXPERIOD;
			cp(output + nout, input + pos, n);
			nout += n;
			nremain -= n;
			pos += n;
		}else{
			in = input + pos;
			period = findperiod(in);
			if(speed >= 2.0f)
				n = period / (speed - 1.0f);
			else{
				n = period;
				nremain = period * (2.0f - speed) / (speed - 1.0f);
			}
			out = output + nout;
			↓ = in;
			↑ = in + period;
			for(i = 0; i < n; i++) /* overlap add */
				*out++ = (*↓++ * (n - i) + *↑++ * i) / n;
			nout += n;
			pos += period + n;
		}
	}while(pos + 2 * MAXPERIOD <= nin);
	cp(input, input + pos, nin -= pos);
}
void main(int i, char *arg[]){
	uchar buf[INSAMPS * 4];
	int p[2], n, more;
	if(i > 2) sysfatal("usage: %s [speed]", arg[0]);
	if(i == 2 && (speed = atof(arg[1])) < 1.0001) sysfatal("I only go fast!");
	if(pipe(p) < 0) sysfatal("pipe: %r");
	n = fork();
	if(n < 0) sysfatal("fork: %r");
	if(n == 0){
		dup(p[1], 0);
		close(p[1]);
		close(p[0]);
		execl("/bin/audio/mp3enc", "mp3enc", "-r", "-m", "m", nil);
		sysfatal("exec: %r");
	}
	close(p[1]);
	do{
		i = read(0, buf, sizeof(buf)) & ~3;
		if(i < 0) sysfatal("read: %r");
		n = i >> 2;
		if(n == 0){/* flush */
			i = nelem(input) - nin;
			while(i--) input[nin + i] = 0;
			i = nout + nin / speed + 0.5f;
			nin += 4 * MAXPERIOD;
			resample();
			if(nout > i) nout = i;
			nin = nremain = 0;
		}else{
			do{
				i -= 4;
				input[nin + (i >> 2)]
					= (s16int)(buf[i | 1] << 8 | buf[i]) + (s16int)(buf[i | 3] << 8 | buf[i | 2]) >> 1;
			}while(i);
			nin += n;
			resample();
		}
		while(nout){
			if(nout <= INSAMPS)
				more = 0;
			else{
				more = nout - INSAMPS;
				nout = INSAMPS;
			}
			for(i = 0; i < nout; i++){/* mp3enc takes big endian */
				buf[i << 1] = output[i] >> 8;
				buf[i << 1 | 1] = output[i];
			}
			if(i && write(p[0], buf, i << 1) != i << 1) sysfatal("write: %r");
			cp(output, output + nout, more);
			nout = more;
		}
	}while(n > 0);
	close(p[0]);
	exits(nil);
}