Implement Unicode compatibility decompositions

Based on patch from Philip Withnall.
https://bugs.freedesktop.org/show_bug.cgi?id=41095

test/api/hb-test.h

diff --git a/test/api/hb-test.h b/test/api/hb-test.h
index d569757..8655f41 100644
--- a/test/api/hb-test.h
+++ b/test/api/hb-test.h
@@ -33,6 +33,7 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <stdio.h>
 
 HB_BEGIN_DECLS
 

test/api/test-unicode.c

diff --git a/test/api/test-unicode.c b/test/api/test-unicode.c
index a420bf3..96c61dd 100644
--- a/test/api/test-unicode.c
+++ b/test/api/test-unicode.c
@@ -786,6 +786,7 @@
 {
   hb_unicode_funcs_t *uf = (hb_unicode_funcs_t *) user_data;
   gunichar a, b, ab;
+  hb_codepoint_t decomposed[HB_UNICODE_MAX_DECOMPOSITION_LEN];
 
 
   /* Test compose() */
@@ -849,6 +850,55 @@
   g_assert (hb_unicode_decompose (uf, 0xCE31, &a, &b) && a == 0xCE20 && b == 0x11B8);
   g_assert (hb_unicode_decompose (uf, 0xCE20, &a, &b) && a == 0x110E && b == 0x1173);
 
+
+  /* Test decompose_compatibility() */
+
+  /* Not decomposable */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x0041, decomposed) == 0);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x1F632, decomposed) == 0);
+
+  /* Singletons */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x00B5, decomposed) == 1 && decomposed[0] == 0x03BC);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x03D6, decomposed) == 1 && decomposed[0] == 0x03C0);
+
+  /* Arabic compatibility */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0xFB54, decomposed) == 1 && decomposed[0] == 0x067B);
+
+  /* Longest decomposition ever */
+  g_assert (18 <= HB_UNICODE_MAX_DECOMPOSITION_LEN);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0xFDFA, decomposed) == 18 && decomposed[17] == 0x0645);
+
+  /* Note: we deliberately don't test characters that have canonical decompositions but no
+   * compatibility decomposition against the decompose_compatibility() function as that we
+   * leave up to implementations (for now). */
+
+  /* Spaces */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2002, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2003, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2004, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2005, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2006, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2008, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2009, decomposed) == 1 && decomposed[0] == 0x0020);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x200A, decomposed) == 1 && decomposed[0] == 0x0020);
+
+  /* Pairs */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x0587, decomposed) == 2 &&
+            decomposed[0] == 0x0565 && decomposed[1] == 0x0582);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2017, decomposed) == 2 &&
+            decomposed[0] == 0x0020 && decomposed[1] == 0x0333);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2025, decomposed) == 2 &&
+            decomposed[0] == 0x002E && decomposed[1] == 0x002E);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2033, decomposed) == 2 &&
+            decomposed[0] == 0x2032 && decomposed[1] == 0x2032);
+
+  /* Triples */
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2026, decomposed) == 3 &&
+            decomposed[0] == 0x002E && decomposed[1] == 0x002E && decomposed[2] == 0x002E);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x2034, decomposed) == 3 &&
+            decomposed[0] == 0x2032 && decomposed[1] == 0x2032 && decomposed[2] == 0x2032);
+  g_assert (hb_unicode_decompose_compatibility (uf, 0x213B, decomposed) == 3 &&
+            decomposed[0] == 0x0046 && decomposed[1] == 0x0041 && decomposed[2] == 0x0058);
 }