import type { Mutable } from '../utils/mutable.js';
import type { Mat } from '../mat/mat.js';
import { isFullyTerminating } from '../cp-node/fs/is-fully-terminating.js';


/**
 * In-place removes trivial 3-prongs from the SAT graph; just to simplify
 * subsequent processing.
 * 
 * These are 3-prongs that are induced by the SAT construction, and do
 * not correspond to actual 3-prongs in the shape.
 * 
 * @param sat the SAT
 */
function removeSatInducedTrivial3Prongs(
        sat: Mat): Mat {

    let { cpNode } = sat;

    let cpStart = cpNode;
    do {
        if (isFullyTerminating(cpNode) ||
            // cpNode.isHoleClosing ||  // hole-closers are fully terminating
            cpNode.next !== cpNode.nextOnCircle) {

            cpNode = cpNode.next;
            continue;
        }

        //-----------------------------
        // Delete the trivial 3-prong
        //-----------------------------
        // console.log('Removing SAT-induced trivial 3-prong');

        const next = cpNode.next;
        const nextNext = next.next;
        const nextNextOC = cpNode.next.nextOnCircle;

        //-------------------------------------------------------------
        // In-place update of links to remove the trivial 3-prong node
        //-------------------------------------------------------------
        (cpNode as Mutable<typeof cpNode>).next = nextNext;
        (cpNode as Mutable<typeof cpNode>).nextOnCircle = nextNextOC;
        (nextNext as Mutable<typeof nextNext>).prev = cpNode;
        (nextNextOC as Mutable<typeof nextNextOC>).prevOnCircle = cpNode;
        //-------------------------------------------------------------

        if (cpStart === next) {
            cpStart = cpNode;  // ...otherwise the loop will never terminate
        }

        cpNode = cpNode.next;
    } while (cpNode !== cpStart)

    // ...otherwise the caller will have a reference to a node that has been removed from the graph
    (sat as Mutable<Mat>).cpNode = cpNode;

    return sat;
}


export { removeSatInducedTrivial3Prongs }