[Perf] nvm_version: resolve an exact, installed version without nvm_ls

An alias of the same name still takes precedence, as it does in `nvm_ls`.

nvm.sh

@@ -913,6 +913,16 @@ nvm_version() {
       PATTERN="stable"
     ;;
   esac
+
+  # Fast path: an exact, fully-qualified version that is installed resolves to
+  # itself, unless an alias of the same name takes precedence. Short-circuiting
+  # it here avoids the `nvm_ls` subprocess pipeline on shell startup; every
+  # other pattern falls through to `nvm_ls` unchanged.
+  if nvm_is_installed_exact_version "${PATTERN}"; then
+    nvm_echo "${PATTERN}"
+    return 0
+  fi
+
   VERSION="$(nvm_ls "${PATTERN}" | command tail -1)"
   case "${VERSION}" in
     system[[:blank:]]*)

test/fast/Unit tests/nvm_version

@@ -2,7 +2,7 @@
 
 die () { echo "$@" ; cleanup ; exit 1; }
 cleanup () {
-  unset -f nvm_ls_current nvm_ls
+  unset -f nvm_ls_current nvm_ls nvm_is_version_installed
 }
 
 : nvm.sh
@@ -48,3 +48,30 @@ try nvm_version foo
 EXPECTED_OUTPUT="N/A"
 [ "_$CAPTURED_STDOUT" = "_$EXPECTED_OUTPUT" ] || die '"nvm_version" did not return N/A when nvm_ls returns nothing'
 [ "_$CAPTURED_EXIT_CODE" = "_3" ] || die '"nvm_version" returning N/A did not exit code with code 3'
+
+# an exact installed version resolves without forking "nvm_ls"
+nvm_is_version_installed() { return 0; }
+nvm_ls() { die '"nvm_version" must not call "nvm_ls" for an exact installed version'; }
+try nvm_version v1.2.3
+EXPECTED_OUTPUT="v1.2.3"
+[ "_$CAPTURED_STDOUT" = "_$EXPECTED_OUTPUT" ] || die '"nvm_version v1.2.3" did not return the version itself; got '"$CAPTURED_STDOUT"
+[ "_$CAPTURED_EXIT_CODE" = "_0" ] || die '"nvm_version v1.2.3" did not exit 0; got '"$CAPTURED_EXIT_CODE"
+
+# an exact version that is not installed still falls back to "nvm_ls"
+nvm_is_version_installed() { return 1; }
+nvm_ls() { echo "pattern: $1"; }
+try nvm_version v1.2.3
+EXPECTED_OUTPUT="pattern: v1.2.3"
+[ "_$CAPTURED_STDOUT" = "_$EXPECTED_OUTPUT" ] || die '"nvm_version v1.2.3" did not fall back to "nvm_ls"; got '"$CAPTURED_STDOUT"
+[ "_$CAPTURED_EXIT_CODE" = "_0" ] || die '"nvm_version v1.2.3" did not exit 0; got '"$CAPTURED_EXIT_CODE"
+
+# only exact, fully-qualified versions take the fast path
+nvm_is_version_installed() { die '"nvm_version" must not call "nvm_is_version_installed" for non-exact versions'; }
+nvm_ls() { echo "pattern: $1"; }
+for PATTERN in v1.2 v1 1.2.3 v1.2.3-rc.1 v1.2.3.4 v1.2.3.; do
+  try nvm_version "$PATTERN"
+  EXPECTED_OUTPUT="pattern: $PATTERN"
+  [ "_$CAPTURED_STDOUT" = "_$EXPECTED_OUTPUT" ] || die "nvm_version $PATTERN did not fall back to nvm_ls; got $CAPTURED_STDOUT"
+  [ "_$CAPTURED_EXIT_CODE" = "_0" ] || die "nvm_version $PATTERN did not exit 0; got $CAPTURED_EXIT_CODE"
+done
+unset -f nvm_is_version_installed