test: add configuration tests and fix bugs

The two new configuation tests, fixed.dfa and float-fixed.dfa verify
that the 'standard' configuration of libpng works without floating point
arithmetic.

Signed-off-by: John Bowler <jbowler@acm.org>

Author: jbowler

contrib/conftest/fixed.dfa

@@ -0,0 +1,15 @@
+# fixed.dfa
+#  Build time configuration of libpng
+#
+# Author: John Bowler
+# Copyright: (c) John Bowler, 2025
+# Usage rights:
+#  To the extent possible under law, the author has waived all copyright and
+#  related or neighboring rights to this work.  This work is published from:
+#  United States.
+#
+# Test the standard libpng configuration without floating point (the internal
+# fixed point implementations are used instead).
+#
+option FLOATING_ARITHMETIC off
+option FLOATING_POINT off

contrib/conftest/float-fixed.dfa

@@ -0,0 +1,14 @@
+# fixed-float.dfa
+#  Build time configuration of libpng
+#
+# Author: John Bowler
+# Copyright: (c) John Bowler, 2025
+# Usage rights:
+#  To the extent possible under law, the author has waived all copyright and
+#  related or neighboring rights to this work.  This work is published from:
+#  United States.
+#
+# Test the standard libpng configuration with the fixed point internal
+# implementation in place of the default floating point
+#
+option FLOATING_ARITHMETIC off

png.c

@@ -2931,6 +2931,30 @@ png_gamma_significant(png_fixed_point gamma_val)
        gamma_val > PNG_FP_1 + PNG_GAMMA_THRESHOLD_FIXED;
 }
 
+#ifndef PNG_FLOATING_ARITHMETIC_SUPPORTED
+/* A local convenience routine. */
+static png_fixed_point
+png_product2(png_fixed_point a, png_fixed_point b)
+{
+   /* The required result is a * b; the following preserves accuracy. */
+#ifdef PNG_FLOATING_ARITHMETIC_SUPPORTED /* Should now be unused */
+   double r = a * 1E-5;
+   r *= b;
+   r = floor(r+.5);
+
+   if (r <= 2147483647. && r >= -2147483648.)
+      return (png_fixed_point)r;
+#else
+   png_fixed_point res;
+
+   if (png_muldiv(&res, a, b, 100000) != 0)
+      return res;
+#endif
+
+   return 0; /* overflow */
+}
+#endif /* FLOATING_ARITHMETIC */
+
 png_fixed_point
 png_reciprocal2(png_fixed_point a, png_fixed_point b)
 {