package pkg

import (
	"bytes"
	"context"
	"errors"
	"os"
	"runtime"
	"slices"

	"hakurei.app/container"
	"hakurei.app/container/check"
	"hakurei.app/container/fhs"
	"hakurei.app/container/std"
	"hakurei.app/message"
)

// ExecContainerPath is an [Artifact] and the [check.Absolute] pathname to make
// it available under in the container.
type ExecContainerPath struct {
	P *check.Absolute
	A Artifact
}

// MustPath returns [ExecContainerPath] for pathname and [Artifact] and panics
// if pathname is not absolute.
func MustPath(pathname string, a Artifact) ExecContainerPath {
	return ExecContainerPath{check.MustAbs(pathname), a}
}

// An execArtifact is an [Artifact] that produces output by running a program
// part of another [Artifact] in a [container] to produce its output.
//
// Methods of execArtifact does not modify any struct field or underlying arrays
// referred to by slices.
type execArtifact struct {
	// Caller-supplied context.
	ctx context.Context
	// Caller-supplied inner read-only bind mounts.
	paths []ExecContainerPath
	// Caller-supplied logging facility, passed through to [container] and used
	// internally to produce verbose output.
	msg message.Msg

	// Number of [Artifact] to concurrently cure. A value of 0 or lower is
	// equivalent to the value returned by [runtime.NumCPU].
	cures int

	// Passed through to [container.Params].
	dir *check.Absolute
	// Passed through to [container.Params].
	env []string
	// Passed through to [container.Params].
	path *check.Absolute
	// Passed through to [container.Params].
	args []string
}

// execNetArtifact is like execArtifact but implements [KnownChecksum] and has
// its resulting container keep the host net namespace.
type execNetArtifact struct {
	checksum Checksum

	execArtifact
}

var _ KnownChecksum = new(execNetArtifact)

// Checksum returns the caller-supplied checksum.
func (a *execNetArtifact) Checksum() Checksum { return a.checksum }

// Kind returns the hardcoded [Kind] constant.
func (a *execNetArtifact) Kind() Kind { return KindExecNet }

// Params is [Checksum] concatenated with [KindExec] params.
func (a *execNetArtifact) Params() []byte {
	return slices.Concat(a.checksum[:], a.execArtifact.Params())
}

// Cure cures the [Artifact] by curing all its dependencies then running the
// container described by the caller. The container retains host networking.
func (a *execNetArtifact) Cure(c *CureContext) error {
	return a.cure(c, true)
}

// NewExec returns a new [Artifact] bounded by ctx, it cures all [Artifact]
// in paths at the specified maximum concurrent cures limit. Specified paths are
// bind mounted read-only in the specified order in the resulting container.
// A private instance of /proc and /dev is made available to the container.
//
// The working and temporary directories are both created and mounted writable
// on /work and /tmp respectively.
//
// If the first path targets [fhs.AbsRoot], it is made writable via an overlay
// mount with writes going to an ephemeral tmpfs bound to the lifetime of the
// container. This is primarily to make it possible for [container] to set up
// mount points targeting paths not available in the [Artifact] backing root,
// and to accommodate poorly written programs that insist on writing to awkward
// paths, it must not be used as scratch space.
//
// If checksum is non-nil, the resulting [Artifact] implements [KnownChecksum]
// and its container runs in the host net namespace.
//
// A cures value of 0 or lower is equivalent to the value returned by
// [runtime.NumCPU].
func NewExec(
	ctx context.Context,
	msg message.Msg,
	cures int,
	checksum *Checksum,

	dir *check.Absolute,
	env []string,
	path *check.Absolute,
	args []string,

	paths ...ExecContainerPath,
) Artifact {
	a := execArtifact{ctx, paths, msg, cures, dir, env, path, args}
	if checksum == nil {
		return &a
	}
	return &execNetArtifact{*checksum, a}
}

// Kind returns the hardcoded [Kind] constant.
func (a *execArtifact) Kind() Kind { return KindExec }

// Params returns paths, executable pathname and args concatenated together.
func (a *execArtifact) Params() []byte {
	var buf bytes.Buffer
	for _, p := range a.paths {
		if p.P != nil {
			buf.WriteString(p.P.String())
		} else {
			buf.WriteString("invalid P\x00")
		}
		if p.A != nil {
			id := Ident(p.A)
			buf.Write(id[:])
		} else {
			buf.WriteString("invalid A\x00")
		}
	}
	buf.WriteByte(0)
	buf.WriteString(a.dir.String())
	buf.WriteByte(0)
	for _, e := range a.env {
		buf.WriteString(e)
	}
	buf.WriteByte(0)
	buf.WriteString(a.path.String())
	buf.WriteByte(0)
	for _, arg := range a.args {
		buf.WriteString(arg)
	}
	return buf.Bytes()
}

// Dependencies returns a slice of all artifacts collected from caller-supplied
// [ExecContainerPath].
func (a *execArtifact) Dependencies() []Artifact {
	artifacts := make([]Artifact, 0, len(a.paths))
	for _, p := range a.paths {
		if p.A != nil {
			artifacts = append(artifacts, p.A)
		}
	}
	return artifacts
}

// Cure cures the [Artifact] by curing all its dependencies then running the
// container described by the caller.
func (a *execArtifact) Cure(c *CureContext) (err error) {
	return a.cure(c, false)
}

// cure is like Cure but allows optional host net namespace. This is used for
// the [KnownChecksum] variant where networking is allowed.
func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
	cures := a.cures
	if cures < 1 {
		cures = runtime.NumCPU()
	}

	paths := make([][2]*check.Absolute, len(a.paths))
	for i, p := range a.paths {
		if p.P == nil || p.A == nil {
			return os.ErrInvalid
		}
		paths[i][1] = p.P
	}

	if len(paths) > 0 {
		type cureArtifact struct {
			// Index of pending Artifact in paths.
			index int
			// Pending artifact.
			a Artifact
		}
		ac := make(chan cureArtifact, len(paths))
		for i, p := range a.paths {
			ac <- cureArtifact{i, p.A}
		}

		type cureRes struct {
			// Index of result in paths.
			index int
			// Cured pathname.
			pathname *check.Absolute
			// Error returned by c.
			err error
		}
		res := make(chan cureRes)

		for i := 0; i < cures; i++ {
			go func() {
				for d := range ac {
					// computing and encoding identifier is expensive
					if a.msg.IsVerbose() {
						a.msg.Verbosef("curing %s...", Encode(Ident(d.a)))
					}

					var cr cureRes
					cr.index = d.index
					cr.pathname, _, cr.err = c.Cure(d.a)
					res <- cr
				}
			}()
		}

		var count int
		errs := make([]error, 0, len(paths))
		for cr := range res {
			count++

			if cr.err != nil {
				errs = append(errs, cr.err)
			} else {
				paths[cr.index][0] = cr.pathname
			}

			if count == len(paths) {
				break
			}
		}
		close(ac)
		if err = errors.Join(errs...); err != nil {
			return
		}
	}

	ctx, cancel := context.WithCancel(a.ctx)
	defer cancel()

	z := container.New(ctx, a.msg)
	z.ForwardCancel = true
	z.SeccompPresets |= std.PresetStrict
	z.ParentPerm = 0700
	z.HostNet = hostNet
	z.Hostname = "cure"
	if z.HostNet {
		z.Hostname = "cure-net"
	}
	z.Uid, z.Gid = (1<<10)-1, (1<<10)-1
	if a.msg.IsVerbose() {
		z.Stdin, z.Stdout, z.Stderr = os.Stdin, os.Stdout, os.Stderr
	}

	z.Dir, z.Env, z.Path, z.Args = a.dir, a.env, a.path, a.args
	z.Grow(len(paths) + 4)
	if len(paths) > 0 && paths[0][1].Is(fhs.AbsRoot) {
		z.OverlayEphemeral(fhs.AbsRoot, paths[0][0])
		paths = paths[1:]
	}
	for _, b := range paths {
		z.Bind(b[0], b[1], 0)
	}
	z.Bind(
		c.GetWorkDir(),
		fhs.AbsRoot.Append("work"),
		std.BindWritable|std.BindEnsure,
	).Bind(
		c.GetTempDir(),
		fhs.AbsTmp,
		std.BindWritable|std.BindEnsure,
	).Proc(fhs.AbsProc).Dev(fhs.AbsDev, true)

	if err = z.Start(); err != nil {
		return
	}
	if err = z.Serve(); err != nil {
		return
	}
	return z.Wait()
}