// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
// All rights reserved.

// operator-digest.go
package expr

import (
	"fmt"
	"io"
)

//-------- digest term

func newDigestTerm(tk *Token) (inst *term) {
	return &term{
		tk:       *tk,
		children: make([]*term, 0, 2),
		position: posInfix,
		priority: priIterOp,
		evalFunc: evalDigest,
	}
}

func evalDigest(ctx ExprContext, opTerm *term) (v any, err error) {
	var leftValue, rightValue any
	var it Iterator
	var item, lastValue any

	if err = opTerm.checkOperands(); err != nil {
		return
	}

	if leftValue, err = opTerm.children[0].compute(ctx); err != nil {
		return
	}

	if it, err = NewIterator(leftValue); err != nil {
		return nil, fmt.Errorf("left operand of DIGEST must be an iterable data-source; got %s", TypeName(leftValue))
	}

	lastValue = nil
	for item, err = it.Next(); err == nil; item, err = it.Next() {
		ctx.SetVar("_", item)
		ctx.SetVar("_index", it.Index())
		ctx.SetVar("_count", it.Count())
		if rightValue, err = opTerm.children[1].compute(ctx); err == nil {
			if rightValue == nil {
				break
			} else {
				lastValue = rightValue
			}
		}
		ctx.DeleteVar("_count")
		ctx.DeleteVar("_index")
		ctx.DeleteVar("_")
		if err != nil {
			break
		}
	}
	if err == io.EOF {
		err = nil
	}
	v = lastValue
	return
}

// init
func init() {
	registerTermConstructor(SymKwDigest, newDigestTerm)
}