implement composition

2026-01-19 19:55:39 +01:00
parent 684b6d78ea
commit c1b8800e82
26 changed files with 91 additions and 68 deletions
--- a/src/commonMain/kotlin/com/infendro/encoding/StreamEncoding.kt
+++ b/src/commonMain/kotlin/com/infendro/encoding/StreamEncoding.kt
@@ -0,0 +1,94 @@
+package com.infendro.encoding
+
+import com.infendro.encoding.exception.IllegalByteException
+import com.infendro.encoding.util.align
+import com.infendro.encoding.util.divCeil
+import com.infendro.encoding.util.lcm
+import com.infendro.encoding.util.log2
+
+class StreamEncoding internal constructor(
+    override val alphabet: ByteArray,
+    private val padding: Byte? = null,
+) : Encoding {
+    init {
+        require(base and (base - 1) == 0)
+    }
+
+    override fun encode(bytes: ByteArray): ByteArray {
+        val bits = log2(base).toInt()
+        val size = (bytes.size * 8).divCeil(bits)
+        val encodedSize = when (padding) {
+            null -> size
+            else -> {
+                val block = lcm(bits, 8) / bits
+                size.align(block)
+            }
+        }
+        val result = ByteArray(encodedSize)
+
+        var r = 0
+        var buffer = 0
+        var bufferBits = 0
+        for (i in bytes.indices) {
+            buffer = (buffer shl 8) + bytes[i]
+            bufferBits += 8
+
+            while (bufferBits >= bits) {
+                val offset = bufferBits - bits
+                val index = (buffer ushr offset)
+                buffer = buffer and ((base - 1) shl offset).inv()
+                bufferBits -= bits
+
+                result[r++] = alphabet[index]
+            }
+        }
+        if (bufferBits > 0) {
+            buffer = buffer shl (bits - bufferBits)
+            val index = buffer
+            result[r++] = alphabet[index]
+        }
+
+        if (padding != null) {
+            while (r < result.size) {
+                result[r++] = padding
+            }
+        }
+
+        return result
+    }
+
+    override fun decode(bytes: ByteArray): ByteArray {
+        val bits = log2(base).toInt()
+        val index = padding?.let { bytes.indexOf(it) } ?: -1
+        val decodedSize = when (index) {
+            -1 -> bytes.size * bits / 8
+            else -> index * bits / 8
+        }
+        val result = ByteArray(decodedSize)
+
+        var r = 0
+        var buffer = 0
+        var bufferBits = 0
+        for (i in bytes.indices) {
+            val byte = bytes[i]
+            if (byte == padding) break
+
+            val index = alphabet.indexOf(byte)
+            if (index == -1) throw IllegalByteException(byte)
+
+            buffer = (buffer shl bits) + index.toByte()
+            bufferBits += bits
+
+            if (bufferBits >= 8) {
+                val offset = bufferBits - 8
+                val character = (buffer ushr offset).toByte()
+                buffer = buffer and (0xFF shl offset).inv()
+                bufferBits -= 8
+
+                result[r++] = character
+            }
+        }
+
+        return result
+    }
+}