package optimizer

// passRegDead eliminates dead stores to REGISTER variables.
// A store is dead if no matching load (lda regVar) follows before A is
// clobbered or a label is reached. This is safe because REGISTER variables
// have a contract: nothing external can observe their memory location.
func passRegDead(lines []asmLine, registerVars map[string]bool) []asmLine {
	var result []asmLine

	for i := 0; i < len(lines); i++ {
		line := lines[i]

		if line.isCode && line.opcode == "sta" && line.operand != "" && registerVars[line.operand] {
			if isRegStoreDead(lines, i+1, line.operand) {
				continue
			}
		}

		result = append(result, line)
	}

	return result
}

// isRegStoreDead scans forward from start to determine whether a store to a
// REGISTER variable operand is dead. Returns true if the store can be removed.
// A store is dead only if no matching lda is found anywhere forward before
// a label or control-flow-ending instruction.
func isRegStoreDead(lines []asmLine, start int, operand string) bool {
	for i := start; i < len(lines); i++ {
		l := lines[i]

		if l.optMarker || l.isComment {
			continue
		}

		if l.isLabel {
			return false
		}

		if l.isCode && isCallSite(l) {
			return false // callee may modify A; conservatively keep the store
		}

		if l.isCode && blocksFlow(l) {
			return true // rts/jmp/brk/rti — execution ends here
		}

		if l.isCode && l.opcode == "lda" && l.operand == operand {
			return false // the value IS read from memory later
		}
	}

	return true // end of block reached with no matching load
}

// blocksFlow returns true for instructions that unconditionally end the current
// execution path: rts, jmp, brk, rti
func blocksFlow(line asmLine) bool {
	switch line.opcode {
	case "rts", "jmp", "brk", "rti":
		return true
	}
	return false
}

// isCallSite returns true for instructions that transfer control to a callee
// that may modify A, X, Y. The store before a call is conservatively kept.
func isCallSite(line asmLine) bool {
	return line.opcode == "jsr"
}