monitor: Add helper to parse simple mode strings

This will be used to extract the resolution and refresh rate from
strings like "1920x1080@60.0" or "1280x720". This aims to replace the
use of the locale dependent sscanf() function.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2902>

src/backends/meta-monitor.c

@@ -2305,3 +2305,50 @@ meta_monitor_set_hdr_metadata (MetaMonitor            *monitor,
 
   return TRUE;
 }
+
+gboolean
+meta_parse_monitor_mode (const char *string,
+                         int        *out_width,
+                         int        *out_height,
+                         float      *out_refresh_rate,
+                         float       fallback_refresh_rate)
+{
+  char *ptr = (char *) string;
+  int width, height;
+  float refresh_rate;
+
+  width = g_ascii_strtoull (ptr, &ptr, 10);
+  if (width == 0)
+    return FALSE;
+
+  if (ptr[0] != 'x')
+    return FALSE;
+  ptr++;
+
+  height = g_ascii_strtoull (ptr, &ptr, 10);
+  if (height == 0)
+    return FALSE;
+
+  if (ptr[0] == '\0')
+    {
+      refresh_rate = fallback_refresh_rate;
+      goto out;
+    }
+
+  if (ptr[0] != '@')
+    return FALSE;
+  ptr++;
+
+  refresh_rate = g_ascii_strtod (ptr, &ptr);
+  if (refresh_rate == 0.0)
+    return FALSE;
+
+  if (ptr[0] != '\0')
+    return FALSE;
+
+out:
+  *out_width = width;
+  *out_height = height;
+  *out_refresh_rate = refresh_rate;
+  return TRUE;
+}

src/backends/meta-monitor.h

@@ -318,4 +318,11 @@ gboolean meta_monitor_set_hdr_metadata (MetaMonitor            *monitor,
                                         MetaOutputHdrMetadata  *metadata,
                                         GError                **error);
 
+META_EXPORT_TEST
+gboolean meta_parse_monitor_mode (const char *string,
+                                  int        *out_width,
+                                  int        *out_height,
+                                  float      *out_refresh_rate,
+                                  float       fallback_refresh_rate);
+
 #endif /* META_MONITOR_H */

src/tests/monitor-util-tests.c

@@ -130,6 +130,85 @@ meta_test_monitor_mode_spec_similar_size (void)
     }
 }
 
+static void
+meta_test_monitor_parse_mode (void)
+{
+  const float fallback_refresh_rate = 60.0;
+  struct {
+    const char *string;
+
+    gboolean expected_pass;
+    int expected_width;
+    int expected_height;
+    float expected_refresh_rate;
+  } test_cases[] = {
+    {
+      .string = "800x600",
+      .expected_pass = TRUE,
+      .expected_width = 800,
+      .expected_height = 600,
+      .expected_refresh_rate = fallback_refresh_rate,
+    },
+    {
+      .string = "1280x720@30",
+      .expected_pass = TRUE,
+      .expected_width = 1280,
+      .expected_height = 720,
+      .expected_refresh_rate = 30.0,
+    },
+    {
+      .string = "1920x1080@120.50",
+      .expected_pass = TRUE,
+      .expected_width = 1920,
+      .expected_height = 1080,
+      .expected_refresh_rate = 120.5,
+    },
+    {
+      .string = "800X600",
+      .expected_pass = FALSE,
+    },
+    {
+      .string = "800x",
+      .expected_pass = FALSE,
+    },
+    {
+      .string = "800x600@",
+      .expected_pass = FALSE,
+    },
+    {
+      .string = "800x600@notanumber",
+      .expected_pass = FALSE,
+    },
+    {
+      .string = "nonsense",
+      .expected_pass = FALSE,
+    },
+  };
+  int i;
+
+  for (i = 0; i < G_N_ELEMENTS (test_cases); i++)
+    {
+      int width, height;
+      float refresh_rate;
+
+      if (meta_parse_monitor_mode (test_cases[i].string,
+                                   &width, &height, &refresh_rate,
+                                   fallback_refresh_rate))
+        {
+          g_assert_true (test_cases[i].expected_pass);
+          g_assert_cmpint (width, ==, test_cases[i].expected_width);
+          g_assert_cmpint (height, ==, test_cases[i].expected_height);
+          g_assert_cmpfloat_with_epsilon (refresh_rate,
+                                          test_cases[i].expected_refresh_rate,
+                                          FLT_EPSILON);
+        }
+      else
+        {
+          g_assert_false (test_cases[i].expected_pass);
+        }
+    }
+}
+
 int
 main (int    argc,
       char **argv)
@@ -138,6 +217,8 @@ main (int    argc,
 
   g_test_add_func ("/backends/monitor/spec/similar-size",
                    meta_test_monitor_mode_spec_similar_size);
+  g_test_add_func ("/backends/monitor/parse-mode",
+                   meta_test_monitor_parse_mode);
 
   return g_test_run ();
 }