62 lines
1.9 KiB
JavaScript
62 lines
1.9 KiB
JavaScript
/* Copyright 2012 Mozilla Foundation
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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import { DecodeStream } from "./decode_stream.js";
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class RunLengthStream extends DecodeStream {
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constructor(str, maybeLength) {
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super(maybeLength);
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this.str = str;
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this.dict = str.dict;
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}
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readBlock() {
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// The repeatHeader has following format. The first byte defines type of run
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// and amount of bytes to repeat/copy: n = 0 through 127 - copy next n bytes
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// (in addition to the second byte from the header), n = 129 through 255 -
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// duplicate the second byte from the header (257 - n) times, n = 128 - end.
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const repeatHeader = this.str.getBytes(2);
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if (!repeatHeader || repeatHeader.length < 2 || repeatHeader[0] === 128) {
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this.eof = true;
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return;
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}
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let buffer;
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let bufferLength = this.bufferLength;
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let n = repeatHeader[0];
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if (n < 128) {
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// copy n bytes
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buffer = this.ensureBuffer(bufferLength + n + 1);
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buffer[bufferLength++] = repeatHeader[1];
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if (n > 0) {
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const source = this.str.getBytes(n);
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buffer.set(source, bufferLength);
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bufferLength += n;
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}
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} else {
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n = 257 - n;
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const b = repeatHeader[1];
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buffer = this.ensureBuffer(bufferLength + n + 1);
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for (let i = 0; i < n; i++) {
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buffer[bufferLength++] = b;
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}
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}
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this.bufferLength = bufferLength;
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}
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}
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export { RunLengthStream };
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