scpt/scpt.go

/*
	Copyright (c) 2021 Arsen Musayelyan

	Permission is hereby granted, free of charge, to any person obtaining a copy
	of this software and associated documentation files (the "Software"), to deal
	in the Software without restriction, including without limitation the rights
	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
	copies of the Software, and to permit persons to whom the Software is
	furnished to do so, subject to the following conditions:

	The above copyright notice and this permission notice shall be included in all
	copies or substantial portions of the Software.
*/

// Package scpt provides an interpreter for my simple applescript-like
// scripting language
package scpt

import (
	"errors"
	"fmt"
	"github.com/alecthomas/participle"
	"github.com/antonmedv/expr"
	"io"
	"reflect"
	"strings"
)

// Vars stores any variables set during script runtime
var Vars = map[string]interface{}{}

// FuncMap is a map of strings mapped to suitable script functions
type FuncMap map[string]func(map[string]interface{}) (interface{}, error)

// Funcs stores the functions allowed for use in a script
var Funcs = FuncMap{
	"str":    toString,
	"num":    parseNumber,
	"bool":   parseBool,
	"break":  setBreakLoop,
	"append": appendArray,
}

// AddFuncs adds all functions from the provided FuncMap into
// the Funcs variable
func AddFuncs(fnMap FuncMap) {
	// Add each function to Funcs
	for name, fn := range fnMap {
		Funcs[name] = fn
	}
}

// AddVars adds all functions from the provided map into
// the Vars variable
func AddVars(varMap map[string]interface{}) {
	for name, val := range varMap {
		// Add each variable to Vars
		Vars[name] = val
	}
}

// Parse uses participle to parse a script from r into a new AST
func Parse(r io.Reader) (*AST, error) {
	// Build parser from empty AST struct with custom lexer
	parser, err := participle.Build(
		&AST{},
		participle.Lexer(scptLexer),
		participle.Elide("Whitespace", "Comment"),
	)
	if err != nil {
		return nil, err
	}
	// Create new empty AST struct
	ast := &AST{}
	// Parse script from provided reader into ast
	err = parser.Parse(r, ast)
	if err != nil {
		return nil, err
	}
	// Return filled AST struct
	return ast, nil
}

// ParseValue parses a Value struct into a go value
func ParseValue(val *Value) (interface{}, error) {
	// Determine which value was provided and return it
	if val.String != nil {
		// Return unquoted string
		return strings.Trim(*val.String, `"`), nil
	} else if val.Bool != nil {
		// Return dereferenced Bool converted to bool
		return bool(*val.Bool), nil
	} else if val.Number != nil {
		// Return dereferenced float
		return *val.Number, nil
	} else if val.SubCmd != nil {
		// Return reference to subcommand
		return val.SubCmd, nil
	} else if val.VarVal != nil {
		// If variable access contains index
		if val.VarVal.Index != nil {
			// Get index value
			index, err := callIfFunc(ParseValue(val.VarVal.Index))
			if err != nil {
				return nil, err
			}
			// Get requested variable and attempt to assert as []interface{}
			slc, ok := Vars[*val.VarVal.Name].([]interface{})
			// If assertion successful
			if ok {
				// Attempt to assert index as a 64-bit float
				indexFlt, ok := index.(float64)
				if !ok {
					return nil, errors.New("array index must be a number")
				}
				// If requested index is out of range, return error
				if int64(len(slc)) <= int64(indexFlt) {
					return nil, fmt.Errorf("index %d is out of range with length %d", *val.VarVal.Index, len(slc))
				}
				// Return value at requested index of requested variable
				return slc[int64(indexFlt)], nil
			} else {
				// If assertion unsuccessful, attempt to assert as a map[interface{}]interface{}
				iMap, ok := Vars[*val.VarVal.Name].(map[interface{}]interface{})
				if !ok {
					return nil, errors.New("variable " + *val.VarVal.Name + " does not exist or is not a map")
				}
				// Attempt to get value at requested key
				val, ok := iMap[index]
				if !ok {
					return nil, fmt.Errorf("index %v does not exist in map", index)
				}
				// Return value at key with no error
				return val, nil
			}
		} else {
			// If index is absent, attempt to get variable value from Vars
			value, ok := Vars[*val.VarVal.Name]
			if !ok {
				return nil, errors.New("variable " + *val.VarVal.Name + " does not exist")
			}
			// Return value with no error
			return value, nil
		}
	} else if val.Expr != nil {
		// If value is an expression, return evaluated expression
		return evalExpr(*val.Expr)
	} else if val.Array != nil {
		// If value is an array, create new nil []interface{}
		var iSlice []interface{}
		// For each value in array
		for _, value := range val.Array {
			// Recursively parse value
			iVal, err := ParseValue(value)
			if err != nil {
				return nil, err
			}
			// Append value to []interface{}
			iSlice = append(iSlice, iVal)
		}
		// Return []interface{]
		return iSlice, nil
	} else if val.Map != nil {
		// If value is a map, create new empty map[interface{}]interface{}
		iMap := map[interface{}]interface{}{}
		// For each value in map
		for _, value := range val.Map {
			// Recursively parse value
			iVal, err := ParseValue(value.Value)
			if err != nil {
				return nil, err
			}
			// Recursively parse key
			iKey, err := ParseValue(value.Key)
			if err != nil {
				return nil, err
			}
			// Set key of map to value
			iMap[iKey] = iVal
		}
		// Return map[interface{}]interface{}
		return iMap, nil
	}
	return nil, nil
}

// Evaluate given expression, returning its value and optionally an error
func evalExpr(expression Expression) (interface{}, error) {
	// Parse value of left side of expression
	left, _ := callIfFunc(ParseValue(expression.Left))
	// If value is string, requote
	if isStr(left) {
		left = quoteStr(left.(string))
	}
	// Create new nil string
	var right string
	// For every right gsegment
	for _, segment := range expression.RightSegs {
		// Parse value of right segment, calling it if it is a function
		rVal, _ := callIfFunc(ParseValue(segment.Right))
		// If value is string, requote
		if isStr(rVal) {
			rVal = quoteStr(rVal)
		}
		// Append right segment to right string
		right = right + fmt.Sprintf(
			" %s %v",
			segment.Op,
			rVal,
		)
	}
	// Create string expression from segments and operator
	exp := fmt.Sprintf(
		"%v %s",
		left,
		right,
	)
	// Compile string expression
	program, err := expr.Compile(strings.ReplaceAll(exp, "^", "**"))
	if err != nil {
		return nil, err
	}
	// Run expression
	out, err := expr.Run(program, Vars)
	if err != nil {
		return nil, err
	}
	// Return expression output value
	return out, nil
}

// Add quotes to an unquoted string
func quoteStr(s interface{}) string {
	// If s is nil
	if s == nil {
		// Return empty quotes
		return `""`
	} else {
		// Otherwise return formatted string using %v (any value)
		return fmt.Sprintf(`"%v"`, s)
	}
}

// Check if i is a string
func isStr(i interface{}) bool {
	if i == nil {
		return true
	}
	// if type of input is string, return true
	if reflect.TypeOf(i).String() == "string" {
		return true
	}
	return false
}

// Call val if it is a function, otherwise pass through return values
func callIfFunc(val interface{}, err error) (interface{}, error) {
	if err != nil {
		return nil, err
	}
	// If val is a pointer to a FuncCall
	if IsFuncCall(val) {
		// Pass through return values of function call
		return CallFunction(val.(*FuncCall))
	}
	// Return given value
	return val, nil
}

// UnwrapArgs takes a slice of Arg structs and returns a map
// storing the argument name and its value. If the argument has
// no name, its key will be an empty string
func UnwrapArgs(args []*Arg) (map[string]interface{}, error) {
	// Create new empty map of strings to any type
	argMap := map[string]interface{}{}
	// For each argument
	for _, arg := range args {
		// Parse value into interface{}
		val, err := ParseValue(arg.Value)
		if err != nil {
			return nil, err
		}
		// If value is function call
		if IsFuncCall(val) {
			// Call function, setting its value as the argument's value
			argMap[arg.Key], err = CallFunction(val.(*FuncCall))
			if err != nil {
				return nil, err
			}
			// Skip further code and start next loop
			continue
		}
		// Set argument value to parsed value
		argMap[arg.Key] = val
	}
	// Return map of arguments
	return argMap, nil
}

// IsFuncCall checks if val is a FuncCall struct
func IsFuncCall(val interface{}) bool {
	// If type of val is a pointer to FuncCall, return true
	if reflect.TypeOf(val) == reflect.TypeOf(&FuncCall{}) {
		return true
	}
	return false
}

// CallFunction executes a given function call in the form of
// a FuncCall struct
func CallFunction(call *FuncCall) (interface{}, error) {
	// Unwrap provided arguments
	argMap, err := UnwrapArgs(call.Args)
	if err != nil {
		return nil, err
	}
	// Attempt to get function from Funcs map
	fn, ok := Funcs[call.Name]
	if !ok {
		return nil, errors.New("no such function: " + call.Name)
	}
	// Return value received from function
	return fn(argMap)
}