Add piet::util module

This is intended for bits of code shared between various backend
implementations.

closes #183

piet-direct2d/src/text.rs

@@ -8,7 +8,7 @@ pub use dwrite::DwriteFactory;
 use std::convert::TryInto;
 
 use piet::kurbo::Point;
-
+use piet::util;
 use piet::{
     Error, Font, FontBuilder, HitTestMetrics, HitTestPoint, HitTestTextPosition, LineMetric, Text,
     TextLayout, TextLayoutBuilder,
@@ -169,8 +169,8 @@ impl TextLayout for D2DTextLayout {
         //
         // TODO ask about text_position, it looks like windows returns last index;
         // can't use the text_position of last index from directwrite, it has an extra code unit.
-        let text_position =
-            count_until_utf16(&self.text, text_position_16).unwrap_or_else(|| self.text.len());
+        let text_position = util::count_until_utf16(&self.text, text_position_16)
+            .unwrap_or_else(|| self.text.len());
 
         HitTestPoint {
             metrics: HitTestMetrics { text_position },
@@ -187,7 +187,7 @@ impl TextLayout for D2DTextLayout {
 
         // Now convert the utf8 index to utf16.
         // This can panic;
-        let idx_16 = count_utf16(&self.text[0..text_position]);
+        let idx_16 = util::count_utf16(&self.text[0..text_position]);
 
         // panic or Result are also fine options for dealing with overflow. Using Option here
         // because it's already present and convenient.
@@ -212,45 +212,6 @@ impl TextLayout for D2DTextLayout {
     }
 }
 
-/// Counts the number of utf-16 code units in the given string.
-/// from xi-editor
-pub(crate) fn count_utf16(s: &str) -> usize {
-    let mut utf16_count = 0;
-    for &b in s.as_bytes() {
-        if (b as i8) >= -0x40 {
-            utf16_count += 1;
-        }
-        if b >= 0xf0 {
-            utf16_count += 1;
-        }
-    }
-    utf16_count
-}
-
-/// returns utf8 text position (code unit offset)
-/// at the given utf-16 text position
-pub(crate) fn count_until_utf16(s: &str, utf16_text_position: usize) -> Option<usize> {
-    let mut utf8_count = 0;
-    let mut utf16_count = 0;
-    #[allow(clippy::explicit_counter_loop)]
-    for &b in s.as_bytes() {
-        if (b as i8) >= -0x40 {
-            utf16_count += 1;
-        }
-        if b >= 0xf0 {
-            utf16_count += 1;
-        }
-
-        if utf16_count > utf16_text_position {
-            return Some(utf8_count);
-        }
-
-        utf8_count += 1;
-    }
-
-    None
-}
-
 #[cfg(test)]
 mod test {
     use super::*;

piet/src/lib.rs

@@ -2,6 +2,9 @@
 
 pub use kurbo;
 
+/// utilities shared by various backends
+pub mod util;
+
 mod color;
 mod conv;
 mod error;

piet/src/util.rs

@@ -0,0 +1,39 @@
+//! Code useful for multiple backends
+
+/// Counts the number of utf-16 code units in the given string.
+/// from xi-editor
+pub fn count_utf16(s: &str) -> usize {
+    let mut utf16_count = 0;
+    for &b in s.as_bytes() {
+        if (b as i8) >= -0x40 {
+            utf16_count += 1;
+        }
+        if b >= 0xf0 {
+            utf16_count += 1;
+        }
+    }
+    utf16_count
+}
+
+/// returns utf8 text position (code unit offset) at the given utf-16 text position
+#[allow(clippy::explicit_counter_loop)]
+pub fn count_until_utf16(s: &str, utf16_text_position: usize) -> Option<usize> {
+    let mut utf8_count = 0;
+    let mut utf16_count = 0;
+    for &b in s.as_bytes() {
+        if (b as i8) >= -0x40 {
+            utf16_count += 1;
+        }
+        if b >= 0xf0 {
+            utf16_count += 1;
+        }
+
+        if utf16_count > utf16_text_position {
+            return Some(utf8_count);
+        }
+
+        utf8_count += 1;
+    }
+
+    None
+}