package ast

import (
	"git.akyoto.dev/cli/q/src/build/errors"
	"git.akyoto.dev/cli/q/src/build/expression"
	"git.akyoto.dev/cli/q/src/build/keyword"
	"git.akyoto.dev/cli/q/src/build/token"
)

// Parse generates an AST from a list of tokens.
func Parse(tokens token.List) (AST, error) {
	tree := make(AST, 0, len(tokens)/64)

	err := EachInstruction(tokens, func(instruction token.List) error {
		node, err := toASTNode(instruction)

		if err == nil && node != nil {
			tree = append(tree, node)
		}

		return err
	})

	return tree, err
}

// toASTNode generates an AST node from an instruction.
func toASTNode(tokens token.List) (Node, error) {
	if tokens[0].Kind == token.Keyword {
		word := tokens[0].Text()

		if word == keyword.Return {
			value := expression.Parse(tokens[1:])
			return &Return{Value: value}, nil
		}

		if keywordHasBlock(word) {
			blockStart := tokens.IndexKind(token.BlockStart)
			blockEnd := tokens.LastIndexKind(token.BlockEnd)

			if blockStart == -1 {
				return nil, errors.New(errors.MissingBlockStart, nil, tokens[0].End())
			}

			if blockEnd == -1 {
				return nil, errors.New(errors.MissingBlockEnd, nil, tokens[len(tokens)-1].End())
			}

			body, err := Parse(tokens[blockStart+1 : blockEnd])

			switch word {
			case keyword.If:
				condition := expression.Parse(tokens[1:blockStart])
				return &If{Condition: condition, Body: body}, err

			case keyword.Loop:
				return &Loop{Body: body}, err
			}
		}

		return nil, errors.New(&errors.KeywordNotImplemented{Keyword: word}, nil, tokens[0].Position)
	}

	expr := expression.Parse(tokens)

	if expr == nil {
		return nil, nil
	}

	switch {
	case IsVariableDefinition(expr):
		if len(expr.Children) < 2 {
			return nil, errors.New(errors.MissingOperand, nil, expr.Token.End())
		}

		name := expr.Children[0].Token
		value := expr.Children[1]
		return &Define{Name: name, Value: value}, nil

	case IsAssignment(expr):
		if len(expr.Children) < 2 {
			return nil, errors.New(errors.MissingOperand, nil, expr.Token.End())
		}

		name := expr.Children[0].Token
		value := expr.Children[1]
		return &Assign{Name: name, Value: value, Operator: expr.Token}, nil

	case IsFunctionCall(expr):
		return &Call{Expression: expr}, nil

	default:
		return nil, errors.New(&errors.InvalidInstruction{Instruction: expr.Token.Text()}, nil, expr.Token.Position)
	}
}

// IsAssignment returns true if the expression is an assignment.
func IsAssignment(expr *expression.Expression) bool {
	return expr.Token.Kind == token.Operator && expr.Token.Bytes[len(expr.Token.Bytes)-1] == '='
}

// IsFunctionCall returns true if the expression is a function call.
func IsFunctionCall(expr *expression.Expression) bool {
	return expr.Token.Kind == token.Operator && expr.Token.Text() == "λ"
}

// IsVariableDefinition returns true if the expression is a variable definition.
func IsVariableDefinition(expr *expression.Expression) bool {
	return expr.Token.Kind == token.Operator && expr.Token.Text() == ":="
}

// keywordHasBlock returns true if the keyword requires a block.
func keywordHasBlock(word string) bool {
	return word == keyword.If || word == keyword.Loop
}