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const { lex } = require('./lexer.js')
let symbols = {}
let throwSyntaxError = () => {
throw new Error('Syntax error: unexpected token')
}
const dieBindingPower = 30
let newSymbol = (type, nud, lbp, led) => {
symbols[type] = {
type,
nud: nud || throwSyntaxError,
lbp,
led: led || throwSyntaxError
}
}
let lexemeToToken = lexeme => {
let token = Object.create(symbols[lexeme.type])
token.value = lexeme.value
return token
}
newSymbol('constant', function() {
return { type: 'constant', value: this.value }
})
newSymbol('(', function(parser) {
const value = parser.expression(1)
parser.match(')')
return value
})
newSymbol(')')
const newDieOperation = (symbol, nud = null) => {
newSymbol(symbol, nud, dieBindingPower, (left, parser) => {
return {
type: symbol,
left: left,
right: parser.expression(dieBindingPower - 1)
}
})
}
newSymbol('d', (parser) => {
return {
type: 'd',
left: { type: 'constant', value: 1 },
right: parser.expression(29)
}
}, 30, (left, parser) => {
return {
type: 'd',
left: left,
right: parser.expression(29)
}
})
newSymbol('E', null, 30, (left, parser) => {
return {
type: 'E',
left: left,
right: parser.expression(29)
}
})
newDieOperation('K')
newSymbol('+', null, 20, (left, parser) => {
return {
type: 'add',
left: left,
right: parser.expression(20)
}
})
newSymbol('-', (parser) => {
return {
type: 'negative',
value: parser.expression(40)
}
}, 20, (left, parser) => {
return {
type: 'subtract',
left: left,
right: parser.expression(20)
}
})
newSymbol('end', null, -1)
const newParser = (tokens) => {
return {
tokens,
currentToken: 0,
token: function() { return this.tokens[this.currentToken] },
advanceToken: function() { this.currentToken++ },
match: function(token) {
if (this.token().type === token) {
this.advanceToken()
} else {
throw throwSyntaxError()
}
},
expression: function(rbp) {
let symbol = this.token()
this.advanceToken()
let left = symbol.nud(this)
while (rbp < this.token().lbp) {
symbol = this.token()
this.advanceToken()
left = symbol.led(left, this)
}
return left
}
}
}
const parse = expressionString => {
const tokens = lex(expressionString).map(lexemeToToken)
tokens.push(symbols.end)
const parser = newParser(tokens)
const expression = parser.expression(0)
parser.match('end')
return expression
}
exports.parse = parse
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