jasonwyatt/KWasm

/*
 * Copyright 2020 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
 * either express or implied. See the License for the specific language governing permissions and
 * limitations under the License.
 */

package kwasm.format.text.instruction

import kwasm.ast.Identifier
import kwasm.ast.Identifier.Label
import kwasm.format.ParseException
import kwasm.format.text.ParseResult
import kwasm.format.text.token.Keyword
import kwasm.format.text.token.Reserved
import kwasm.format.text.token.Token

/**
 * Parses an [Identifier.Label] from the tokens.
 *
 * From [the docs](https://webassembly.github.io/spec/core/text/instructions.html#labels):
 *
 * ```
 *   labelI ::= v:id    => compose({ labels v }, I) (if v ∉ I.labels)
 *              ϵ       => compose({ labels (ϵ) }, I)
 * ```
 *
 * When the [Token] at [startIndex] is not an [kwasm.format.text.token.Identifier], this function:
 * * returns an *empty* identifier, if and only if the it is not a [Keyword] nor a [Reserved], or
 * * throws a [ParseException].
 *
 * **Note:** Composition will be left up to the interpreter.
 */
fun List<Token>.parseLabel(startIndex: Int): ParseResult<Label> {
    val idString = when (val identifier = getOrNull(startIndex)) {
        is kwasm.format.text.token.Identifier -> identifier.value
        is Reserved -> throw ParseException("Invalid identifier", identifier.context)
        else -> null
    }

    val parseLength = if (idString.isNullOrEmpty()) 0 else 1
    val identifierAst = Label(idString)

    return ParseResult(identifierAst, parseLength)
}