examples/mandelbrot: ANSI 256-color background per pixel

examples/mandelbrot/mandelbrot.ww

@@ -1,4 +1,4 @@
-// mandelbrot — ASCII Mandelbrot set, printed via libc.
+// mandelbrot — ANSI-color Mandelbrot set, printed via libc.
 //
 // What this proves:
 //   1. ww does f64 arithmetic (mul, add, sub, div) end to end.
@@ -7,9 +7,11 @@
 //      a real .so at runtime — `ldd mandelbrot` shows libc.so.6 as a
 //      NEEDED entry.
 //
-// We use libc's `write` (a thin syscall wrapper) rather than `putchar`
-// because the runtime here doesn't call __libc_start_main, so stdio
-// state stays uninitialised and putchar's buffer is never flushed.
+// Each pixel is a colored space rendered via xterm 256-color background
+// escapes (\e[48;5;Nm). We use libc's `write` (a thin syscall wrapper)
+// rather than `putchar` because the runtime here doesn't call
+// __libc_start_main, so stdio state stays uninitialised and putchar's
+// buffer is never flushed.
 //
 // Build: see ./Makefile. No cc — pure ww toolchain.
 
@@ -36,14 +38,70 @@ fn iterate(cr: f64, ci: f64) i32 = {
 	return MAXI;
 };
 
-// Pick an ASCII glyph by escape iteration count. Inside the set
-// (n == MAXI) renders as space — the classic look.
-fn shade(n: i32) i32 = {
-	if (n >= MAXI) { return ' ': i32; };
-	let chars: str = " .:-=+*#%@";
-	let idx:   i32 = (n * chars.len) / MAXI;
-	if (idx >= chars.len) { idx = chars.len - 1; };
-	return chars[idx]: i32;
+// Pick an xterm-256 color for an escape count. Inside the set is black;
+// outside cycles blue → cyan → green → yellow → red.
+fn palette(n: i32) i32 = {
+	if (n >= MAXI) { return 16; };	// black, inside set
+	let q: i32 = (n * 12) / MAXI;
+	if (q ==  0) { return  17; };	// dark blue
+	if (q ==  1) { return  18; };
+	if (q ==  2) { return  19; };
+	if (q ==  3) { return  21; };	// bright blue
+	if (q ==  4) { return  33; };
+	if (q ==  5) { return  45; };
+	if (q ==  6) { return  51; };	// cyan
+	if (q ==  7) { return  50; };
+	if (q ==  8) { return  46; };	// green
+	if (q ==  9) { return 154; };
+	if (q == 10) { return 226; };	// yellow
+	if (q == 11) { return 208; };	// orange
+	return 196;			// red
+};
+
+// Write `n` as decimal into buf at off; return new offset.
+fn writedec(buf: *u8, off: i32, n: i32) i32 = {
+	if (n == 0) { buf[off] = '0': u8; return off + 1; };
+	let tmp: [4]u8;
+	let v:   i32 = n;
+	let i:   i32 = 0;
+	for (v > 0) {
+		tmp[i] = ((v % 10) + 48): u8;
+		v = v / 10;
+		i += 1;
+	};
+	let cnt: i32 = i;
+	let j:   i32 = 0;
+	for (j < cnt) {
+		i -= 1;
+		buf[off + j] = tmp[i];
+		j += 1;
+	};
+	return off + cnt;
+};
+
+// Emit "\e[48;5;<color>m " (set background + the pixel) into buf.
+fn emitcell(buf: *u8, off: i32, color: i32) i32 = {
+	buf[off + 0] = 27u8;
+	buf[off + 1] = '[': u8;
+	buf[off + 2] = '4': u8;
+	buf[off + 3] = '8': u8;
+	buf[off + 4] = ';': u8;
+	buf[off + 5] = '5': u8;
+	buf[off + 6] = ';': u8;
+	let p: i32 = writedec(buf, off + 7, color);
+	buf[p + 0] = 'm': u8;
+	buf[p + 1] = ' ': u8;
+	return p + 2;
+};
+
+// Emit "\e[0m\n" — restore default colors, end the row.
+fn emitendrow(buf: *u8, off: i32) i32 = {
+	buf[off + 0] = 27u8;
+	buf[off + 1] = '[': u8;
+	buf[off + 2] = '0': u8;
+	buf[off + 3] = 'm': u8;
+	buf[off + 4] = 10u8;	// '\n'
+	return off + 5;
 };
 
 export fn main() i32 = {
@@ -61,21 +119,23 @@ export fn main() i32 = {
 	let dx:   f64 = (xmax - xmin) / (W: f64);
 	let dy:   f64 = (ymax - ymin) / (H: f64);
 
-	// Compose the picture into a row buffer, then flush per row to
-	// avoid one libc call per character.
-	let row: [128]u8;
+	// Each pixel is ~12 bytes (\e[48;5;NNNm ); plus 5 for the reset
+	// + newline. 2048 bytes per row is comfortably above the worst
+	// case of W=78.
+	let row: [2048]u8;
 	let y:   i32 = 0;
 	for (y < H) {
 		let ci:  f64 = ymin + (y: f64) * dy;
+		let off: i32 = 0;
 		let x:   i32 = 0;
 		for (x < W) {
 			let cr: f64 = xmin + (x: f64) * dx;
 			let n:  i32 = iterate(cr, ci);
-			row[x] = (shade(n)): u8;
+			off = emitcell(row.ptr, off, palette(n));
 			x += 1;
 		};
-		row[W] = 10u8;	// '\n'
-		c_write(1, row.ptr: *void, (W + 1): u64);
+		off = emitendrow(row.ptr, off);
+		c_write(1, row.ptr: *void, off: u64);
 		y += 1;
 	};
 	return 0;