fix: faster toString for integrity (#75)

Note that the ordering of hashes in the result varies slightly.

lib/index.js

@@ -3,7 +3,7 @@
 const crypto = require('crypto')
 const MiniPass = require('minipass')
 
-const SPEC_ALGORITHMS = ['sha256', 'sha384', 'sha512']
+const SPEC_ALGORITHMS = ['sha512', 'sha384', 'sha256']
 const DEFAULT_ALGORITHMS = ['sha512']
 
 // TODO: this should really be a hardcoded list of algorithms we support,
@@ -186,6 +186,39 @@ class Hash {
   }
 }
 
+function integrityHashToString (toString, sep, opts, hashes) {
+  const toStringIsNotEmpty = toString !== ''
+
+  let shouldAddFirstSep = false
+  let complement = ''
+
+  const lastIndex = hashes.length - 1
+
+  for (let i = 0; i < lastIndex; i++) {
+    const hashString = Hash.prototype.toString.call(hashes[i], opts)
+
+    if (hashString) {
+      shouldAddFirstSep = true
+
+      complement += hashString
+      complement += sep
+    }
+  }
+
+  const finalHashString = Hash.prototype.toString.call(hashes[lastIndex], opts)
+
+  if (finalHashString) {
+    shouldAddFirstSep = true
+    complement += finalHashString
+  }
+
+  if (toStringIsNotEmpty && shouldAddFirstSep) {
+    return toString + sep + complement
+  }
+
+  return toString + complement
+}
+
 class Integrity {
   get isIntegrity () {
     return true
@@ -201,15 +234,24 @@ class Integrity {
 
   toString (opts) {
     let sep = opts?.sep || ' '
+    let toString = ''
+
     if (opts?.strict) {
       // Entries must be separated by whitespace, according to spec.
       sep = sep.replace(/\S+/g, ' ')
+
+      for (const hash of SPEC_ALGORITHMS) {
+        if (this[hash]) {
+          toString = integrityHashToString(toString, sep, opts, this[hash])
+        }
+      }
+    } else {
+      for (const hash of Object.keys(this)) {
+        toString = integrityHashToString(toString, sep, opts, this[hash])
+      }
     }
-    return Object.keys(this).map(k => {
-      return this[k].map(hash => {
-        return Hash.prototype.toString.call(hash, opts)
-      }).filter(x => x.length).join(sep)
-    }).filter(x => x.length).join(sep)
+
+    return toString
   }
 
   concat (integrity, opts) {