1 files changed, 30 insertions, 0 deletions
diff --git a/image/png/paeth.ha b/image/png/paeth.ha
new file mode 100644
index 0000000..4abc373
--- /dev/null
+++ b/image/png/paeth.ha
@@ -0,0 +1,30 @@
+// Based on Go's implementation; BSD license
+use math;
+
+fn applypaeth(dec: *decoder) void = {
+	const bpp = dec.bpp;
+	let a = 0i32, b = 0i32, c = 0i32, pa = 0i32, pb = 0i32, pc = 0i32;
+	for (let i = 0z; i < bpp; i += 1) {
+		a = 0;
+		c = 0;
+		for (let j = i; j < len(dec.cr); j += bpp) {
+			b = dec.pr[j]: i32;
+			pa = b - c;
+			pb = a - c;
+			pc = math::absi32(pa + pb): i32;
+			pa = math::absi32(pa): i32;
+			pb = math::absi32(pb): i32;
+			if (pa <= pb && pa <= pc) {
+				void; // no-op
+			} else if (pb <= pc) {
+				a = b;
+			} else {
+				a = c;
+			};
+			a += dec.cr[j]: i32;
+			a &= 0xff;
+			dec.cr[j] = a: u8;
+			c = b;
+		};
+	};
+};