prime numbers

.editorconfig

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+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+
+[*.{js,ts,json}]
+charset = utf-8
+indent_style = space
+indent_size = 2

.eslintignore

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+!.meta
+
+# Protected or generated
+.git
+.vscode
+
+# When using npm
+node_modules/*
+
+# Configuration files
+babel.config.js
+jest.config.js

.eslintrc

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+{
+  "root": true,
+  "parser": "@typescript-eslint/parser",
+  "parserOptions": {
+    "project": "./tsconfig.json",
+    "ecmaFeatures": {
+      "jsx": true
+    },
+    "ecmaVersion": 2020,
+    "sourceType": "module"
+  },
+  "extends": "@exercism/eslint-config-typescript",
+  "env": {
+    "jest": true
+  },
+  "overrides": [
+    {
+      "files": [".meta/proof.ci.ts", ".meta/exemplar.ts", "*.test.ts"],
+      "excludedFiles": ["custom.test.ts"],
+      "extends": "@exercism/eslint-config-typescript/maintainers"
+    }
+  ]
+}

src/prime.class.ts

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+export class Prime {
+  private static values: number[] = [2, 3, 5, 7];
+
+  private static lastKnown(): number {
+    return Prime.values.slice(-1)[0];
+  }
+
+  private static searchNextKnownPrime(number: number, from = 0, to: number = Prime.values.length): number {
+    // this is a binary search
+    // basically we are splitting the list into two halves and
+    // then discard the half that does not contain the input number
+    // and call the function again with the remaining half
+    // but instead of always compiling a new array we just use indices of base array
+
+    const length = to - from;
+
+    if (length > 1) {
+      const middle = from + Math.floor(length / 2);
+
+      return Prime.values[middle] < number
+        ? Prime.searchNextKnownPrime(number, from, middle + 1)
+        : Prime.searchNextKnownPrime(number, middle, to)
+    }
+
+    return Prime.values[from] > number ? Prime.values[from] : 0;
+  }
+
+  private static findNextUnknown(): number {
+    let number = Prime.lastKnown();
+
+    do {
+      number += 2;
+    } while (!Prime.isPrime(number));
+
+    Prime.values.push(number);
+    return number;
+  }
+
+  private static next(number: number): number {
+    if (number < Prime.lastKnown()) {
+      // if input is a prime number just return the next one in our list
+      const index = Prime.values.indexOf(number);
+      if (index !== -1) {
+        return Prime.values[index + 1];
+      }
+
+      // input is not a prime number, so we have to find the next larger entry
+      return Prime.searchNextKnownPrime(number);
+    }
+
+    // input is larger than last known prime number in our list
+    // so we need to brute force the next one
+    return Prime.findNextUnknown();
+  }
+
+  public static isPrime(number: number): boolean {
+    if (number <= 1 || !Number.isInteger(number)) {
+      return false;
+    }
+
+    if (number <= Prime.lastKnown()) {
+      return Prime.values.indexOf(number) !== -1;
+    }
+
+    const limit = Math.sqrt(number);
+
+    if (limit <= Prime.lastKnown()) {
+      for (const prime of Prime.values) {
+        if (prime > limit) {
+          break;
+        }
+
+        if (number % prime === 0) {
+          return false;
+        }
+      }
+      return true;
+    }
+
+    // calculate new prime numbers up to limit
+    while (Prime.findNextUnknown() < limit);
+
+    // now we should have a sufficient list
+    // to determine if input is a prime number
+    return Prime.isPrime(number);
+  }
+
+  public static isGoodWeak(number: number): boolean {
+    if (number <= 2 || !Prime.isPrime(number)) {
+      return false;
+    }
+
+    const prev = Prime.values[Prime.values.indexOf(number) - 1];
+    const next = Prime.lastKnown() === number
+      ? Prime.findNextUnknown()
+      : Prime.values[Prime.values.indexOf(number) + 1];
+
+    return Math.pow(number, 2) > prev * next;
+  }
+
+  public static isGood(number: number): boolean {
+    if (number <= 2 || !Prime.isPrime(number)) {
+      return false;
+    }
+
+    const square = Math.pow(number, 2);
+    const pos = Prime.values.indexOf(number);
+    let upper = number;
+    for (let i = pos - 1; i >= 0; i -= 1) {
+      const lower = Prime.values[i];
+      upper = Prime.next(upper);
+
+      if (square < lower * upper) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  public static factors(number: number): number[] {
+    if (Prime.isPrime(number)) {
+      return [number];
+    }
+
+    const factors: number[] = [];
+
+    let rest = number;
+    let prime = 2;
+
+    do {
+      if (rest % prime === 0) {
+        rest = rest / prime;
+        factors.push(prime);
+      }
+      else {
+        prime = Prime.next(prime);
+      }
+    } while (1 < rest && prime <= rest);
+
+    return factors;
+  }
+}

src/test.ts

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+import { Prime } from './prime.class.ts';
+
+for (let i = 0; i < 1000; i++) {
+  if (Prime.isPrime(i)) {
+    console.log(`${i}${Prime.isGood(i) ? ' good' : ''}`);
+  }
+}

tsconfig.json

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+{
+  "compilerOptions": {
+    /* Basic Options */
+    "target": "esnext" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */,
+    "module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */,
+    "lib": [
+      "esnext",
+      "es2016",
+      "es2017"
+    ] /* Specify library files to be included in the compilation. */,
+    // "allowJs": true,                       /* Allow javascript files to be compiled. */
+    // "checkJs": true,                       /* Report errors in .js files. */
+    // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
+    // "declaration": true,                   /* Generates corresponding '.d.ts' file. */
+    // "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
+    // "sourceMap": true,                        /* Generates corresponding '.map' file. */
+    // "outFile": "./",                       /* Concatenate and emit output to single file. */
+    // "outDir": "./build",                      /* Redirect output structure to the directory. */
+    // "rootDirs": ["./"],                    /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
+    // "composite": true,                     /* Enable project compilation */
+    // "removeComments": true,                /* Do not emit comments to output. */
+    "noEmit": true /* Do not emit outputs. */,
+    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
+    // "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
+    "isolatedModules": true /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */,
+    // "noEmitOnError": true,                    /* Do not emit outputs when compilation fails. */
+
+    /* Strict Type-Checking Options */
+    "strict": true /* Enable all strict type-checking options. */,
+    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
+    // "strictNullChecks": true,              /* Enable strict null checks. */
+    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
+    // "strictBindCallApply": true,           /* Enable strict 'bind', 'call', and 'apply' methods on functions. */
+    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
+    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
+    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */
+
+    /* Additional Checks */
+    // "noUnusedLocals": true,                /* Report errors on unused locals. */
+    "noUnusedParameters": true /* Report errors on unused parameters. */,
+    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
+    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */
+
+    /* Module Resolution Options */
+    "moduleResolution": "node" /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */,
+    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
+    // "paths": { "~src/*": ["./src/*"] },                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
+    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
+    // "typeRoots": [],                       /* List of folders to include type definitions from. */
+    // "types": [],                           /* Type declaration files to be included in compilation. */
+    "allowSyntheticDefaultImports": true /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */,
+    "esModuleInterop": true /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
+    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */
+
+    /* Source Map Options */
+    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
+    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
+    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
+    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */
+
+    /* Experimental Options */
+    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
+    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
+  },
+  "compileOnSave": true,
+  "include": ["src/*", ".meta/*"],
+  "exclude": ["node_modules"]
+}