Add implementation

Implement the CTC beam search and update README

Signed-off-by: Yihong Wang <[email protected]>

.npmignore

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+node_modules/
+.npmignore
+src/
+.vscode/
+package-lock.json

README.md

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 # beam-search
-Implement the Connectionist Temporal Classification(CTC) beam search in JavaScript
+Implement the Connectionist Temporal Classification(CTC) beam search in
+JavaScript. The input is log probabilities of an array. The length of the
+array is the number of CTC slots. Each item in the array contains an array of
+log probabilities to each characters, including blank character. Usually
+the blank character is the last one. The implementation doesn't support NGram
+now. But it's one of the todos.
+
+## Usage
+The following code is used to handle English CTC results:
+``` javascript
+const { CTCBeamSearch, EN_VOCABULARY } = require('ctc-beam-search');
+const bs = new CTCBeamSearch(EN_VOCABULARY);
+const data = ....; // log probabilities
+const results = bs.search(data, 5); // beam width = 5
+// dump the first result to console as a string
+console.log(results[0].convertToStr(EN_VOCABULARY));
+```
+
+The `EN_VOCABULARY` is like this:
+``` javascript
+const { Vocabulary } = require('ctc-beam-search');
+const engV = new Vocabulary({ ' ': 0,
+                              'a': 1,
+                              'b': 2,
+                              'c': 3,
+                              'd': 4,
+                              'e': 5,
+                              'f': 6,
+                              'g': 7,
+                              'h': 8,
+                              'i': 9,
+                              'j': 10,
+                              'k': 11,
+                              'l': 12,
+                              'm': 13,
+                              'n': 14,
+                              'o': 15,
+                              'p': 16,
+                              'q': 17,
+                              'r': 18,
+                              's': 19,
+                              't': 20,
+                              'u': 21,
+                              'v': 22,
+                              'w': 23,
+                              'x': 24,
+                              'y': 25,
+                              'z': 26,
+                              '\'': 27,
+                            }, 28);
+```
+You can create you own Vocabulary.

package.json

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+{
+  "name": "ctc-beam-search",
+  "version": "0.1.0",
+  "description": "Implement Connectionist Temporal Classificiation(CTC) beam search in JavaScript",
+  "main": "dist/index.js",
+  "scripts": {
+    "build": "tsc"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/yhwang/beam-search.git"
+  },
+  "keywords": [
+    "CTC",
+    "beam",
+    "search"
+  ],
+  "author": "[email protected]",
+  "license": "Apache-2.0",
+  "bugs": {
+    "url": "https://github.com/yhwang/beam-search/issues"
+  },
+  "homepage": "https://github.com/yhwang/beam-search#readme",
+  "devDependencies": {
+    "@types/node": "^12.12.17",
+    "tslint": "^5.20.1",
+    "typescript": "3.5.3"
+  }
+}

src/constants.ts

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+// Blank index (-) in the CHAR_MAP
+export const EN_BLANK_INDEX = 28;
+// Character list for English
+export const EN_CHARS = ' abcdefghijklmnopqrstuvwxyz\'';
+// Character map for English. string as key and index as value
+export const EN_CHAR_MAP: {[key: string]: number} = {};
+export const EPSILON = 1e-5;
+export const IS_NODE = typeof(window) === 'undefined';
+
+// Initialize the EN_CHAR_MAP
+for (let index = 0, len = EN_CHARS.length; index < len; index++) {
+  EN_CHAR_MAP[EN_CHARS.charAt(index)] = index;
+}