1 files changed, 50 insertions, 7 deletions

diff --git a/src/moe/anita/mai/buffer/MemBuf.java b/src/moe/anita/mai/buffer/MemBuf.java
index a2bc477..922e624 100644
--- a/src/moe/anita/mai/buffer/MemBuf.java
+++ b/src/moe/anita/mai/buffer/MemBuf.java
@@ -1,29 +1,72 @@
 package moe.anita.mai.buffer;
 
+import java.nio.charset.StandardCharsets;
 import java.util.Objects;
 
 /**
- * Buffer backed entirely by an in-memory string.
+ * An in-memory UTF-8 source buffer.
+ *
+ * Offsets and lengths are measured in UTF-8 bytes.
  */
 public final class MemBuf implements Buffer {
 
-	private final String data;
+	private final byte[] data;
 
-	public MemBuf(String data) { this.data = Objects.requireNonNull(data);
+	public MemBuf(String source) {
+		Objects.requireNonNull(source);
+		data = source.getBytes(StandardCharsets.UTF_8);
 	}
 
 	@Override
 	public int length() {
-		return data.length();
+		return data.length;
 	}
 
 	@Override
-	public char get(int index) {
-		return data.charAt(index);
+	public int get(int offset) {
+		int first = Byte.toUnsignedInt(data[offset]);
+
+		if ((first & 0x80) == 0) {
+			return first;
+		}
+
+		if ((first & 0xE0) == 0xC0) {
+			return ((first & 0x1F) << 6) | (Byte.toUnsignedInt(data[offset + 1]) & 0x3F);
+		}
+
+		if ((first & 0xF0) == 0xE0) {
+			return ((first & 0x0F) << 12)
+				| ((Byte.toUnsignedInt(data[offset + 1]) & 0x3F) << 6)
+				| (Byte.toUnsignedInt(data[offset + 2]) & 0x3F);
+		}
+
+		return ((first & 0x07) << 18)
+			|  ((Byte.toUnsignedInt(data[offset + 1]) & 0x3F) << 12)
+			| ((Byte.toUnsignedInt(data[offset + 2]) & 0x3F) << 6)
+			| (Byte.toUnsignedInt(data[offset + 3]) & 0x3F);
+	}
+
+	@Override
+	public int width(int offset) {
+		int first = Byte.toUnsignedInt(data[offset]);
+
+		if ((first & 0x80) == 0) {
+			return 1;
+		}
+
+		if ((first & 0xE0) == 0xC0) {
+			return 2;
+		}
+
+		if ((first & 0xF0) == 0xE0) {
+			return 3;
+		}
+
+		return 4;
 	}
 
 	@Override
 	public CharSequence slice(int start, int end) {
-		return data.subSequence(start, end);
+		return new String(data, start, end - start, StandardCharsets.UTF_8);
 	}
 }