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package org.danilopianini.symmetricmatrix

import kotlin.math.max
import kotlin.math.min

/**
 * Indexing functions for symmetric matrices.
 */
abstract class AbstractSymmetricMatrix : SymmetricMatrix {
    protected val internalSize = size * (size + 1) / 2

    /**
     * Given an [index] in the internal representation of the matrix, return the corresponding indices.
     */
    protected fun indicesFromIndex(index: Int): Pair {
        require(index in 0 until internalSize) {
            "Invalid index: $index, not in [0, $internalSize)"
        }
        var i = 0
        while (i * (i + 1) / 2 <= index) {
            i++
        }
        i-- // Adjust back to the correct row
        val j = index - i * (i + 1) / 2
        return Pair(min(i, j), max(i, j))
    }

    /**
     * Given two indices ([i], [j]), return the corresponding index in the internal representation of the matrix.
     */
    protected fun indexOf(
        i: Int,
        j: Int,
    ): Int =
        max(i, j).let { max ->
            require(max in 0 until size) {
                "Invalid index: ($i, $j), max($i, $j) not in [0, $size)"
            }
            max * (max + 1) / 2 +
                min(i, j).also { min ->
                    require(min in 0 until size) {
                        "Invalid index: ($i, $j), min($i, $j) not in [0, $size)"
                    }
                }
        }

    override fun row(i: Int): List = (0 until size).map { j -> get(i, j) }

    override fun column(j: Int): List = (0 until size).map { i -> get(i, j) }

    override fun toString(): String = "SymmetricMatrix($size, $size)"

    override fun iterator(): Iterator = (0 until size).asSequence().flatMap { row(it).asSequence() }.iterator()
}