// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package mvs implements Minimal Version Selection.
// See https://research.swtch.com/vgo-mvs.
package mvs

import (
	"fmt"
	"slices"
	"sort"
	"sync"

	"cmd/go/internal/par"

	"golang.org/x/mod/module"
)

name: down2
A: B2 E2
B1:
B2: C2 F2
C1:
D1:
C2: D2 E2
D2: B2
E2: D2
E1:
F1:
build A:        A B2 C2 D2 E2 F2
downgrade A F1: A B1 C1 D1 E1 F1

# https://research.swtch.com/vgo-mvs#algorithm_4:
# “[D]owngrades are constrained to only downgrade packages, not also upgrade
# them; if an upgrade before downgrade is needed, the user must ask for it
# explicitly.”
#

	sumdbWrongServer = sumdb.NewServer(sumdbWrongOps)
)

// proxyHandler serves the Go module proxy protocol.
// See the proxy section of https://research.swtch.com/vgo-module.
func proxyHandler(w http.ResponseWriter, r *http.Request) {
	if !strings.HasPrefix(r.URL.Path, "/mod/") {
		http.NotFound(w, r)
		return
	}
	path := r.URL.Path[len("/mod/"):]

		// commits would use the previous name instead of the new one.
		//
		// That's especially problematic if the original pseudo-version starts with
		// v0.0.0-, as was the case for all pseudo-versions during vgo development,
		// since a v0.0.0- pseudo-version has lower precedence than pretty much any
		// tagged version.
		//
		// In practice, we're only looking up by hash during initial conversion of a