implement composition

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

@@ -0,0 +1,43 @@
+package com.infendro.encoding
+
+import com.infendro.encoding.exception.IllegalByteException
+import com.infendro.encoding.util.BigInt
+import com.infendro.encoding.util.log2
+import kotlin.math.ceil
+
+class NumericEncoding internal constructor(
+    override val alphabet: ByteArray,
+) : Encoding {
+    override fun encode(bytes: ByteArray): ByteArray {
+        if (bytes.isEmpty()) return byteArrayOf()
+
+        val size = ceil(bytes.size * 8 / log2(base)).toInt()
+        val result = ByteArray(size)
+
+        var i = result.size
+        var x = BigInt.from(bytes)
+        while (!x.isZero()) {
+            val (quotient, remainder) = x.divRem(base)
+            x = quotient
+            result[--i] = alphabet[remainder]
+        }
+
+        return result.copyOfRange(i, result.size)
+    }
+
+    override fun decode(bytes: ByteArray): ByteArray {
+        if (bytes.isEmpty()) return byteArrayOf()
+
+        var value = BigInt.zero
+        for (i in 0..<bytes.size) {
+            val byte = bytes[i]
+
+            val index = alphabet.indexOf(byte)
+            if (index == -1) throw IllegalByteException(byte)
+
+            value = value.timesAdd(base, index)
+        }
+
+        return value.toByteArray()
+    }
+}