Add LEB128 encoding

2 files changed, 122 insertions, 0 deletions

diff --git a/src/main/java/org/shedlang/compiler/backends/wasm/structure/BinaryWriter.java b/src/main/java/org/shedlang/compiler/backends/wasm/structure/BinaryWriter.java
new file mode 100644
index 00000000..d5600a5c
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/backends/wasm/structure/BinaryWriter.java
@@ -0,0 +1,57 @@
+package org.shedlang.compiler.backends.wasm.structure;
+
+import java.nio.ByteBuffer;
+
+public class BinaryWriter {
+    private ByteBuffer buffer = ByteBuffer.allocate(1024);
+
+    public void writeByte(byte value) {
+        ensureSpareCapacity(1);
+        buffer.put(value);
+    }
+
+    public void writeBytes(byte[] bytes) {
+        ensureSpareCapacity(bytes.length);
+        buffer.put(bytes, 0, bytes.length);
+    }
+
+    public ReservedBytes reserve(int length) {
+        var position = buffer.position();
+        advance(length);
+        return new ReservedBytes(position);
+    }
+
+    private void ensureSpareCapacity(int length) {
+        ensureCapacity(buffer.position() + length);
+    }
+
+    private void ensureCapacity(int minCapacity) {
+        if (minCapacity > buffer.capacity()) {
+            var newCapacity = buffer.capacity() * 2;
+            while (minCapacity > newCapacity) {
+                newCapacity *= 2;
+            }
+            var newBuffer = ByteBuffer.allocate(newCapacity);
+            buffer.flip();
+            newBuffer.put(buffer);
+            buffer = newBuffer;
+        }
+    }
+
+    private void advance(int length) {
+        ensureSpareCapacity(length);
+        buffer.position(buffer.position() + length);
+    }
+
+    public byte[] toBytes() {
+        var result = new byte[buffer.position()];
+        buffer.get(0, result);
+        return result;
+    }
+
+    public static class ReservedBytes {
+        public ReservedBytes(int position) {
+
+        }
+    }
+}
diff --git a/src/main/java/org/shedlang/compiler/backends/wasm/structure/Leb128Encoding.java b/src/main/java/org/shedlang/compiler/backends/wasm/structure/Leb128Encoding.java
new file mode 100644
index 00000000..bcd7de5f
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/backends/wasm/structure/Leb128Encoding.java
@@ -0,0 +1,65 @@
+package org.shedlang.compiler.backends.wasm.structure;
+
+public class Leb128Encoding {
+    private Leb128Encoding() {
+    }
+
+    static void encodeUnsignedInt32(int value, BinaryWriter writer) {
+        while (true) {
+            var nextByte = value & 0x7f;
+            value >>>= 7;
+            if (value == 0) {
+                writer.writeByte((byte) nextByte);
+                return;
+            } else {
+                writer.writeByte((byte) (nextByte | 0x80));
+            }
+        }
+    }
+
+    static void encodePaddedUnsignedInt32(int value, BinaryWriter writer) {
+        for (var byteIndex = 0; byteIndex < 4; byteIndex++) {
+            var nextByte = value & 0x7f;
+            writer.writeByte((byte) (nextByte | 0x80));
+            value >>>= 7;
+        }
+        writer.writeByte((byte) value);
+    }
+
+    static void encodeSignedInt32(int value, BinaryWriter writer) {
+        if (value >= 0) {
+            while (true) {
+                if ((value >> 6) == 0) {
+                    writer.writeByte((byte) value);
+                    return;
+                } else {
+                    writer.writeByte((byte) ((value & 0x7f) | 0x80));
+                    value >>= 7;
+                }
+            }
+        } else {
+            while (true) {
+                if ((value >> 6) == -1) {
+                    writer.writeByte((byte) (value & 0x7f));
+                    return;
+                } else {
+                    writer.writeByte((byte) ((value & 0x7f) | 0x80));
+                    value >>= 7;
+                }
+            }
+        }
+    }
+
+    static void encodePaddedSignedInt32(int value, BinaryWriter writer) {
+        if (value > 0) {
+            encodePaddedUnsignedInt32(value, writer);
+        } else {
+            for (var byteIndex = 0; byteIndex < 4; byteIndex++) {
+                var nextByte = value & 0x7f;
+                writer.writeByte((byte) (nextByte | 0x80));
+                value >>= 7;
+            }
+            writer.writeByte((byte) (value & 0x7f));
+        }
+    }
+}