initial commit

src/commonMain/kotlin/com/infendro/encoding/Encoding.kt

@@ -0,0 +1,92 @@
+package com.infendro.encoding
+
+import com.infendro.encoding.util.lcm
+import com.infendro.encoding.util.log2
+import kotlin.math.pow
+
+sealed class Encoding {
+    abstract fun encode(
+        bytes: ByteArray,
+    ): ByteArray
+
+    abstract fun decode(
+        bytes: ByteArray,
+    ): ByteArray
+
+    protected fun encode(
+        bytes: ByteArray,
+        characters: Array<Byte>,
+        padding: Byte? = null,
+    ): ByteArray {
+        val length = log2(characters.size).toInt()
+        val mask = (2.0.pow(length) - 1).toUInt()
+
+        val encoded = mutableListOf<Byte>()
+
+        var buffer = 0U
+        var bits = 0
+
+        for (byte in bytes) {
+            buffer = (buffer shl 8) + byte.toUByte()
+            bits += 8
+
+            while (bits >= length) {
+                val offset = bits - length
+                val index = (buffer shr offset).toInt()
+                buffer = buffer and (mask shl offset).inv()
+                bits -= length
+
+                encoded.add(characters[index])
+            }
+        }
+        if (bits > 0) {
+            buffer = buffer shl (length - bits)
+            val index = buffer.toInt()
+            encoded.add(characters[index])
+        }
+
+        if (padding != null) {
+            val block = lcm(length, 8) / length
+            while (encoded.size % block != 0) {
+                encoded.add(padding)
+            }
+        }
+
+        return encoded.toByteArray()
+    }
+
+    protected fun decode(
+        bytes: ByteArray,
+        characters: Array<Byte>,
+        padding: Byte? = null,
+    ): ByteArray {
+        val length = log2(characters.size).toInt()
+        val mask = 0xFFU
+
+        val decoded = mutableListOf<Byte>()
+
+        var buffer = 0U
+        var bits = 0
+
+        for (byte in bytes) {
+            if (byte == padding) break
+
+            val index = characters.indexOf(byte)
+            if (index == -1) throw Exception()
+
+            buffer = (buffer shl length) + index.toUByte()
+            bits += length
+
+            if (bits >= 8) {
+                val offset = bits - 8
+                val character = (buffer shr offset).toByte()
+                buffer = buffer and (mask shl offset).inv()
+                bits -= 8
+
+                decoded.add(character)
+            }
+        }
+
+        return decoded.toByteArray()
+    }
+}