90 lines
3.1 KiB
JavaScript
90 lines
3.1 KiB
JavaScript
/**
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* @module create-prefix-transform-stream
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* @author Toru Nagashima
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* @copyright 2016 Toru Nagashima. All rights reserved.
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* See LICENSE file in root directory for full license.
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*/
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"use strict"
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//------------------------------------------------------------------------------
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// Requirements
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//------------------------------------------------------------------------------
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const stream = require("stream")
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//------------------------------------------------------------------------------
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// Helpers
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//------------------------------------------------------------------------------
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const ALL_BR = /\n/g
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/**
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* The transform stream to insert a specific prefix.
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*
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* Several streams can exist for the same output stream.
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* This stream will insert the prefix if the last output came from other instance.
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* To do that, this stream is using a shared state object.
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*
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* @private
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*/
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class PrefixTransform extends stream.Transform {
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/**
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* @param {string} prefix - A prefix text to be inserted.
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* @param {object} state - A state object.
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* @param {string} state.lastPrefix - The last prefix which is printed.
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* @param {boolean} state.lastIsLinebreak -The flag to check whether the last output is a line break or not.
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*/
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constructor(prefix, state) {
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super()
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this.prefix = prefix
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this.state = state
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}
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/**
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* Transforms the output chunk.
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*
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* @param {string|Buffer} chunk - A chunk to be transformed.
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* @param {string} _encoding - The encoding of the chunk.
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* @param {function} callback - A callback function that is called when done.
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* @returns {void}
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*/
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_transform(chunk, _encoding, callback) {
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const prefix = this.prefix
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const nPrefix = `\n${prefix}`
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const state = this.state
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const firstPrefix =
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state.lastIsLinebreak ? prefix :
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(state.lastPrefix !== prefix) ? "\n" :
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/* otherwise */ ""
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const prefixed = `${firstPrefix}${chunk}`.replace(ALL_BR, nPrefix)
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const index = prefixed.indexOf(prefix, Math.max(0, prefixed.length - prefix.length))
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state.lastPrefix = prefix
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state.lastIsLinebreak = (index !== -1)
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callback(null, (index !== -1) ? prefixed.slice(0, index) : prefixed)
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}
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}
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//------------------------------------------------------------------------------
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// Public API
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//------------------------------------------------------------------------------
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/**
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* Create a transform stream to insert the specific prefix.
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*
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* Several streams can exist for the same output stream.
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* This stream will insert the prefix if the last output came from other instance.
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* To do that, this stream is using a shared state object.
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*
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* @param {string} prefix - A prefix text to be inserted.
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* @param {object} state - A state object.
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* @param {string} state.lastPrefix - The last prefix which is printed.
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* @param {boolean} state.lastIsLinebreak -The flag to check whether the last output is a line break or not.
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* @returns {stream.Transform} The created transform stream.
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*/
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module.exports = function createPrefixTransform(prefix, state) {
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return new PrefixTransform(prefix, state)
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}
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