diff --git a/sub_crates/text_encoding/src/utf8.rs b/sub_crates/text_encoding/src/utf8.rs
index bc5e326..169479b 100644
--- a/sub_crates/text_encoding/src/utf8.rs
+++ b/sub_crates/text_encoding/src/utf8.rs
@@ -7,44 +7,332 @@ use core;
 use {DecodeError, DecodeResult, EncodeResult};
 
 pub fn encode_from_str<'a>(input: &str, output: &'a mut [u8]) -> EncodeResult<'a> {
-    let copy_len = {
-        if output.len() >= input.len() {
-            input.len()
-        } else {
-            let mut i = output.len();
-            while !input.is_char_boundary(i) {
-                i -= 1;
-            }
-            i
-        }
-    };
-
-    output[..copy_len].copy_from_slice(input[..copy_len].as_bytes());
-
-    Ok((copy_len, &output[..copy_len]))
+    let cl = copy_len(input.as_bytes(), output.len());
+    output[..cl].copy_from_slice(input[..cl].as_bytes());
+    Ok((cl, &output[..cl]))
 }
 
 pub fn decode_to_str<'a>(input: &[u8], output: &'a mut [u8]) -> DecodeResult<'a> {
+    // Find how much of the data is valid utf8.
     let valid_up_to = match core::str::from_utf8(input) {
         Ok(text) => text.len(),
-        Err(e) => {
-            if e.valid_up_to() > 0 {
-                e.valid_up_to()
-            } else {
-                return Err(DecodeError {
-                    error_range: (0, 1), // TODO: search for the next starting byte to get the range.
-                    output_bytes_written: 0,
-                });
-            }
-        }
+        Err(e) => e.valid_up_to(),
     };
 
-    let (in_consumed, out_slice) = encode_from_str(
-        unsafe { core::str::from_utf8_unchecked(&input[..valid_up_to]) },
-        output,
-    ).unwrap();
+    // Copy over what we can.
+    let bytes_copied = copy_len(&input[..valid_up_to], output.len());
+    output[..bytes_copied].copy_from_slice(&input[..bytes_copied]);
 
-    Ok((in_consumed, unsafe {
-        core::str::from_utf8_unchecked(out_slice)
+    // Determine if there's an error.
+    if bytes_copied < output.len() && bytes_copied == valid_up_to && valid_up_to < input.len() {
+        let trailing_bytes = input.len() - valid_up_to;
+        let byte = input[valid_up_to];
+        // First we check if we're truncated.  If we are, then don't error
+        // yet, because we want to provide the full byte range of the error.
+        let is_truncated = ((byte & 0b11100000) == 0b11000000 && trailing_bytes < 2)
+            || ((byte & 0b11110000) == 0b11100000 && trailing_bytes < 3)
+            || ((byte & 0b11111000) == 0b11110000 && trailing_bytes < 4);
+        if !is_truncated {
+            // Find the byte range of the error by finding the next valid
+            // starting byte (or reaching end of input).
+            let mut i = valid_up_to + 1;
+            while i < input.len()
+                && ((input[i] & 0b11000000) == 0b10000000 || (input[i] & 0b11111000) == 0b11111000)
+            {
+                i += 1;
+            }
+            // Return the error.
+            return Err(DecodeError {
+                error_range: (valid_up_to, i),
+                output_bytes_written: bytes_copied,
+            });
+        }
+    }
+
+    // No error, return success.
+    Ok((bytes_copied, unsafe {
+        core::str::from_utf8_unchecked(&output[..bytes_copied])
     }))
 }
+
+/// Calculates how many bytes should be copied from input to output given
+/// their lengths and the content of input.  Specifically, it calculates
+/// the maximum amount that can be copied without incompletely copying
+/// any multi-byte codepoints.
+///
+/// Input is assumed to be valid and complete utf8 (i.e. could be turned
+/// directly into a &str).
+#[inline(always)]
+fn copy_len(input: &[u8], output_len: usize) -> usize {
+    if output_len >= input.len() {
+        input.len()
+    } else {
+        let mut i = output_len;
+        while i > 0 && (input[i] & 0b11000000) == 0b10000000 {
+            i -= 1;
+        }
+        i
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn encode_01() {
+        let text = "こんにちは！";
+        let mut buf = [0u8; 2];
+        let (consumed_count, encoded) = encode_from_str(text, &mut buf).unwrap();
+        assert_eq!(consumed_count, 0);
+        assert_eq!(encoded, &[]);
+    }
+
+    #[test]
+    fn encode_02() {
+        let text = "こんにちは！";
+        let mut buf = [0u8; 3];
+        let (consumed_count, encoded) = encode_from_str(text, &mut buf).unwrap();
+        assert_eq!(consumed_count, 3);
+        assert_eq!(encoded, &[0xE3, 0x81, 0x93]);
+    }
+
+    #[test]
+    fn encode_03() {
+        let text = "こんにちは！";
+        let mut buf = [0u8; 5];
+        let (consumed_count, encoded) = encode_from_str(text, &mut buf).unwrap();
+        assert_eq!(consumed_count, 3);
+        assert_eq!(encoded, &[0xE3, 0x81, 0x93]);
+    }
+
+    #[test]
+    fn decode_01() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！"
+        let mut buf = [0u8; 2];
+        let (consumed_count, decoded) = decode_to_str(&data, &mut buf).unwrap();
+        assert_eq!(consumed_count, 0);
+        assert_eq!(decoded, "");
+    }
+
+    #[test]
+    fn decode_02() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！"
+        let mut buf = [0u8; 3];
+        let (consumed_count, decoded) = decode_to_str(&data, &mut buf).unwrap();
+        assert_eq!(consumed_count, 3);
+        assert_eq!(decoded, "こ");
+    }
+
+    #[test]
+    fn decode_03() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！"
+        let mut buf = [0u8; 5];
+        let (consumed_count, decoded) = decode_to_str(&data, &mut buf).unwrap();
+        assert_eq!(consumed_count, 3);
+        assert_eq!(decoded, "こ");
+    }
+
+    #[test]
+    fn decode_04() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC,
+        ]; // "こんにちは！" with last byte chopped off.
+        let mut buf = [0u8; 64];
+        let (consumed_count, decoded) = decode_to_str(&data, &mut buf).unwrap();
+        assert_eq!(consumed_count, 15);
+        assert_eq!(decoded, "こんにちは");
+    }
+
+    #[test]
+    fn decode_05() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF,
+        ]; // "こんにちは！" with last 2 bytes chopped off.
+        let mut buf = [0u8; 64];
+        let (consumed_count, decoded) = decode_to_str(&data, &mut buf).unwrap();
+        assert_eq!(consumed_count, 15);
+        assert_eq!(decoded, "こんにちは");
+    }
+
+    #[test]
+    fn decode_error_01() {
+        let data = [
+            0b10000000, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3,
+            0x81, 0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the first char (continuing code unit).
+        let mut buf = [0u8; 2];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (0, 3),
+                output_bytes_written: 0,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_02() {
+        let data = [
+            0xE3, 0x81, 0xE3, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81, 0xAF,
+            0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the first code point (too few continuing code units).
+        let mut buf = [0u8; 2];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (0, 2),
+                output_bytes_written: 0,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_03() {
+        let data = [
+            0xE3, 0x81, 0x93, 0b10000000, 0x82, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3,
+            0x81, 0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (continuing code unit).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 6),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_04() {
+        let data = [
+            0xE3, 0x81, 0x93, 0b10000000, 0x82, 0x93, 0b10000000, 0x81, 0xAB, 0b10000000, 0x81,
+            0xA1, 0xE3, 0x81, 0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (lots of continuing code units).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 12),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_05() {
+        let data = [
+            0xE3, 0x81, 0x93, 0b11111000, 0x82, 0x93, 0x93, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81,
+            0xA1, 0xE3, 0x81, 0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (invalid bit pattern).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 8),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_06() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xED, 0xA0, 0x80, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (beginning of surrogate range).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 6),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_07() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xED, 0xBF, 0xBF, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81,
+            0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (end of surrogate range).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 6),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_08() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xF4, 0x90, 0x80, 0x80, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3,
+            0x81, 0xAF, 0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (out of unicode range).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 7),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_09() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xC0, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81, 0xAF,
+            0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (byte == 0xC0).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 5),
+                output_bytes_written: 3,
+            })
+        );
+    }
+
+    #[test]
+    fn decode_error_10() {
+        let data = [
+            0xE3, 0x81, 0x93, 0xC1, 0x93, 0xE3, 0x81, 0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81, 0xAF,
+            0xEF, 0xBC, 0x81,
+        ]; // "こんにちは！" with an error on the second code point (byte == 0xC1).
+        let mut buf = [0u8; 64];
+        let error = decode_to_str(&data, &mut buf);
+        assert_eq!(
+            error,
+            Err(DecodeError {
+                error_range: (3, 5),
+                output_bytes_written: 3,
+            })
+        );
+    }
+}