Squashed 'external/stb/stb/' content from commit c39c7023e

git-subtree-dir: external/stb/stb
git-subtree-split: c39c7023ebb833ce099750fe35509aca5662695e

tests/test_png_paeth.c

@@ -0,0 +1,47 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+// Reference Paeth filter as per PNG spec
+static int ref_paeth(int a, int b, int c)
+{
+   int p = a + b - c;
+   int pa = abs(p-a);
+   int pb = abs(p-b);
+   int pc = abs(p-c);
+   if (pa <= pb && pa <= pc) return a;
+   if (pb <= pc) return b;
+   return c;
+}
+
+// Optimized Paeth filter
+static int opt_paeth(int a, int b, int c)
+{
+   int thresh = c*3 - (a + b);
+   int lo = a < b ? a : b;
+   int hi = a < b ? b : a;
+   int t0 = (hi <= thresh) ? lo : c;
+   int t1 = (thresh <= lo) ? hi : t0;
+   return t1;
+}
+
+int main()
+{
+   // Exhaustively test the functions match for all byte inputs a, b,c in [0,255]
+   for (int i = 0; i < (1 << 24); ++i) {
+      int a = i & 0xff;
+      int b = (i >> 8) & 0xff;
+      int c = (i >> 16) & 0xff;
+
+      int ref = ref_paeth(a, b, c);
+      int opt = opt_paeth(a, b, c);
+      if (ref != opt) {
+         fprintf(stderr, "mismatch at a=%3d b=%3d c=%3d: ref=%3d opt=%3d\n", a, b, c, ref, opt);
+         return 1;
+      }
+   }
+
+   printf("all ok!\n");
+   return 0;
+}
+
+// vim:sw=3:sts=3:et