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| 1 | +/* Copyright © 2025 Apeleg Limited. All rights reserved. |
| 2 | + * |
| 3 | + * Permission to use, copy, modify, and distribute this software for any |
| 4 | + * purpose with or without fee is hereby granted, provided that the above |
| 5 | + * copyright notice and this permission notice appear in all copies. |
| 6 | + * |
| 7 | + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH |
| 8 | + * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY |
| 9 | + * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, |
| 10 | + * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM |
| 11 | + * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR |
| 12 | + * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR |
| 13 | + * PERFORMANCE OF THIS SOFTWARE. |
| 14 | + */ |
| 15 | + |
| 16 | +import { $subtype, $type } from './constants.js'; |
| 17 | +import type { IMediaTypeNegotiationStrategy } from './negotiateMediaTypeFactory.js'; |
| 18 | +import type { TMediaType } from './parseMediaType.js'; |
| 19 | +import { wm } from './utils.js'; |
| 20 | + |
| 21 | +/** |
| 22 | + * Find parameters that overlap between two media types, excluding the `q` |
| 23 | + * parameter. |
| 24 | + * |
| 25 | + * @internal |
| 26 | + * @param a - Candidate media type (typically from `Accept` header). |
| 27 | + * @param b - Available media type (server-provided). |
| 28 | + * @returns Array of matching `[name, value]` parameter pairs. |
| 29 | + */ |
| 30 | +const findOverlappingParams = (a: TMediaType, b: TMediaType) => { |
| 31 | + return a[1].filter((aparam) => { |
| 32 | + return ( |
| 33 | + aparam[0] !== 'q' && |
| 34 | + b[1].some((bparam) => { |
| 35 | + return aparam[0] === bparam[0] && aparam[1] === bparam[1]; |
| 36 | + }) |
| 37 | + ); |
| 38 | + }); |
| 39 | +}; |
| 40 | + |
| 41 | +/** |
| 42 | + * Implements the default HTTP content negotiation strategy, ranking candidates |
| 43 | + * according to the rules outlined in RFC 7231 §5.3.2. |
| 44 | + * |
| 45 | + * This function takes pre-parsed and pre-sorted lists of available (server) and |
| 46 | + * acceptable (client) media types and finds the single best match from the |
| 47 | + * available list. The selection process follows a strict, multi-stage ranking |
| 48 | + * algorithm: |
| 49 | + * |
| 50 | + * 1. **Filtering:** It first identifies all potential matches by comparing |
| 51 | + * each acceptable media range against all available types. A match can be |
| 52 | + * an exact type/subtype match, a subtype wildcard match (e.g., `text/*`) |
| 53 | + * or a full wildcard match (`*` + `/*`). |
| 54 | + * 2. **Quality Value (`q`):** It retains only the candidates that match the |
| 55 | + * highest q-value present among all potential matches. All others are |
| 56 | + * discarded. |
| 57 | + * 3. **Specificity Tie-Breaking:** If multiple candidates remain, they are |
| 58 | + * ranked by specificity in the following order of preference: |
| 59 | + * a. Full type/subtype (e.g., `application/json`) is preferred over |
| 60 | + * wildcard subtypes. |
| 61 | + * b. A wildcard subtype (e.g., `text/*`) is preferred over the full |
| 62 | + * wildcard (`*` + `/*`). |
| 63 | + * c. The number of matching non-q parameters. A media range with more |
| 64 | + * matching parameters to an available type is preferred. |
| 65 | + * d. Server preference. As a final tie-breaker, the original order of the |
| 66 | + * `availableMediaTypes` array is used, preferring the one that |
| 67 | + * appeared earliest. |
| 68 | + * |
| 69 | + * @internal |
| 70 | + * @param availableMediaTypes - A readonly array of pre-parsed and normalised |
| 71 | + * media types supported by the server, in order of server preference. |
| 72 | + * @param acceptableMediaTypes - A readonly array of pre-parsed media types |
| 73 | + * from the client's `Accept` header, pre-sorted by q-value in descending |
| 74 | + * order. |
| 75 | + * @param qMap - A map that holds the q-value (weight) for each acceptable |
| 76 | + * media type, used for efficient lookups. |
| 77 | + * @returns The best-matching normalised media type from the |
| 78 | + * `parsedAvailableTypes` array, or `null` if no suitable match is found. |
| 79 | + */ |
| 80 | +const defaultStrategy = ((availableMediaTypes, acceptableMediaTypes, qMap) => { |
| 81 | + const map = wm< |
| 82 | + (typeof acceptableMediaTypes)[number], |
| 83 | + (typeof availableMediaTypes)[number] |
| 84 | + >(); |
| 85 | + |
| 86 | + // First pass: remove media types that aren't possible |
| 87 | + const overlappingTypes = acceptableMediaTypes.filter( |
| 88 | + (acceptableMediaType) => { |
| 89 | + const possible: (typeof availableMediaTypes)[number][] = []; |
| 90 | + availableMediaTypes.forEach((availableMediaType) => { |
| 91 | + if (acceptableMediaType[0] === availableMediaType[0]) { |
| 92 | + possible.push(availableMediaType); |
| 93 | + } else if ( |
| 94 | + acceptableMediaType[$subtype] === '*' && |
| 95 | + acceptableMediaType[$type] === availableMediaType[$type] |
| 96 | + ) { |
| 97 | + possible.push(availableMediaType); |
| 98 | + } else if ( |
| 99 | + acceptableMediaType[$type] === '*' && |
| 100 | + acceptableMediaType[$subtype] === '*' |
| 101 | + ) { |
| 102 | + possible.push(availableMediaType); |
| 103 | + } |
| 104 | + }); |
| 105 | + |
| 106 | + possible.sort((a, b) => { |
| 107 | + const overlappingA = findOverlappingParams( |
| 108 | + acceptableMediaType, |
| 109 | + a, |
| 110 | + ).length; |
| 111 | + const overlappingB = findOverlappingParams( |
| 112 | + acceptableMediaType, |
| 113 | + b, |
| 114 | + ).length; |
| 115 | + |
| 116 | + return overlappingB - overlappingA; |
| 117 | + }); |
| 118 | + |
| 119 | + if (possible.length) { |
| 120 | + map.set(acceptableMediaType, possible[0]); |
| 121 | + return true; |
| 122 | + } |
| 123 | + |
| 124 | + return false; |
| 125 | + }, |
| 126 | + ); |
| 127 | + |
| 128 | + if (overlappingTypes.length === 0) { |
| 129 | + return null; |
| 130 | + } |
| 131 | + |
| 132 | + // Second pass: keep highest preference only |
| 133 | + const highestQ = qMap.get(overlappingTypes[0])!; |
| 134 | + for (let i = 1; i < overlappingTypes.length; i++) { |
| 135 | + const q = qMap.get(overlappingTypes[i])!; |
| 136 | + if (q !== highestQ) { |
| 137 | + overlappingTypes.splice(i); |
| 138 | + break; |
| 139 | + } |
| 140 | + } |
| 141 | + |
| 142 | + // Now, find the type with the highest specificity |
| 143 | + overlappingTypes.sort((a, b) => { |
| 144 | + if (a[$type] === '*' && b[$type] !== '*') { |
| 145 | + return 1; |
| 146 | + } else if (a[$type] !== '*' && b[$type] === '*') { |
| 147 | + return -1; |
| 148 | + } else if (a[$subtype] === '*' && b[$subtype] !== '*') { |
| 149 | + return 1; |
| 150 | + } else if (a[$subtype] !== '*' && b[$subtype] === '*') { |
| 151 | + return -1; |
| 152 | + } |
| 153 | + |
| 154 | + const availableA = map.get(a)!; |
| 155 | + const availableB = map.get(b)!; |
| 156 | + |
| 157 | + const overlappingA = findOverlappingParams(a, availableA).length; |
| 158 | + const overlappingB = findOverlappingParams(b, availableB).length; |
| 159 | + |
| 160 | + const result = overlappingB - overlappingA; |
| 161 | + if (result !== 0) { |
| 162 | + return result; |
| 163 | + } |
| 164 | + |
| 165 | + // If everything is equal, prefer server order |
| 166 | + return ( |
| 167 | + availableMediaTypes.indexOf(availableA) - |
| 168 | + availableMediaTypes.indexOf(availableB) |
| 169 | + ); |
| 170 | + }); |
| 171 | + |
| 172 | + const highestRanked = overlappingTypes[0]; |
| 173 | + const availableType = map.get(highestRanked)!; |
| 174 | + |
| 175 | + return availableType; |
| 176 | +}) satisfies IMediaTypeNegotiationStrategy; |
| 177 | + |
| 178 | +export default defaultStrategy; |
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