package capsule

import (
	"crypto/hmac"
	"crypto/sha256"
	"errors"
	"fmt"
	"io"
	"time"

	datekeys "g.activething.com/go/DateKeys"
	"g.activething.com/go/DateKeys/extension"
)

// Sink receives the files of a format 3 capsule (spec §56, §63 steps 17 and
// 18). Open calls Begin once the head is validated, then Create for each
// file in the order of the head, and then Commit, only after every check of
// step 17 has passed. After any failure that follows a successful Begin, a
// failure of Commit included, it calls Abort, once; a Begin that fails
// cleans up after itself. Nothing a Sink receives before Commit may be
// presented as valid: write to a temporary place and publish it in Commit.
type Sink interface {
	// Begin receives the validated head. Its files are h.Files.
	Begin(h *Head) error
	// Create returns the writer of file i of the head. Open closes it after
	// writing its Size bytes, before checking its SHA-256. Size, Start and
	// End are declared, not received: a Sink must not reserve memory or disk
	// by them (spec §57).
	Create(i int) (io.WriteCloser, error)
	// Commit publishes the files: step 18.
	Commit() error
	// Abort discards everything the Sink received.
	Abort()
}

// ErrSinkRequired is the error of Open for a format 3 capsule without
// OpenOptions.Sink: its content is several files, which one io.Writer cannot
// receive. It is an error of the caller, with no normative code: Open returns
// it right after step 2, before any request, and never reports the capsule
// as ErrUnsupportedVersion, because it is valid (spec §70).
var ErrSinkRequired = errors.New("capsule: a format 3 capsule holds files: OpenOptions.Sink is required")

// errWriterRequired is the error of Open for a capsule of format 1 or 2 with
// a nil dst.
var errWriterRequired = errors.New("capsule: a capsule of format 1 or 2 holds one content: dst is required")

// plainReader reads the plaintext of PAYLOAD_AGE and counts its bytes.
type plainReader struct {
	r io.Reader
	n uint64
}

func (p *plainReader) Read(b []byte) (int, error) {
	n, err := p.r.Read(b)
	p.n += uint64(n)
	return n, err
}

// plaintextFailure is the error of a read of the plaintext that stopped
// before it wanted: a failure of age, or a plaintext that ends before P.
func plaintextFailure(err error, n, p uint64) error {
	if err == io.EOF {
		return fmt.Errorf("capsule: PAYLOAD_AGE: the plaintext is %d bytes, shorter than P = %d: %w", n, p, datekeys.ErrIntegrity)
	}
	return classify("PAYLOAD_AGE", ageStreamFailure, err)
}

// readN reads exactly n bytes of the plaintext. Its memory grows with the
// bytes received, not with n, a length that BODY declares (spec §57). Not
// io.ReadFull: it would turn the io.ErrUnexpectedEOF of a truncated STREAM
// into a short read.
func readN(r *plainReader, n int, p uint64) ([]byte, error) {
	var buf []byte
	part := make([]byte, min(n, 32<<10))
	for len(buf) < n {
		m, err := r.Read(part[:min(n-len(buf), len(part))])
		buf = append(buf, part[:m]...)
		if len(buf) == n {
			break
		}
		if err != nil {
			return nil, plaintextFailure(err, r.n, p)
		}
	}
	return buf, nil
}

// drain reads the rest of the plaintext to EOF. A failure of age, or a
// plaintext whose length is not P, prevails over the failure of any substep
// of step 17 (spec §63).
func drain(r *plainReader, p uint64) error {
	buf := make([]byte, 32<<10)
	for {
		_, err := r.Read(buf)
		switch {
		case err == io.EOF && r.n != p:
			return fmt.Errorf("capsule: PAYLOAD_AGE: the plaintext is %d bytes, not P = %d: %w", r.n, p, datekeys.ErrIntegrity)
		case err == io.EOF:
			return nil
		case err != nil:
			return classify("PAYLOAD_AGE", ageStreamFailure, err)
		}
	}
}

// copyFile writes the next size bytes of the plaintext to w, hashing them.
func copyFile(w io.Writer, r *plainReader, size, p uint64) ([]byte, error) {
	sum := sha256.New()
	buf := make([]byte, 32<<10)
	for left := size; left > 0; {
		m := uint64(len(buf))
		if m > left {
			m = left
		}
		n, err := r.Read(buf[:m])
		if n > 0 {
			sum.Write(buf[:n])
			if _, werr := w.Write(buf[:n]); werr != nil {
				return nil, &sinkError{werr}
			}
			left -= uint64(n)
		}
		if left > 0 && err != nil {
			return nil, plaintextFailure(err, r.n, p)
		}
	}
	return sum.Sum(nil), nil
}

// refused is the error of OpenOptions.Accept: every check of step 17 passed,
// and the caller refused to publish the files.
type refused struct{ err error }

func (e *refused) Error() string { return e.err.Error() }
func (e *refused) Unwrap() error { return e.err }

// sinkError is a failure of the caller's Sink, not of the capsule.
type sinkError struct{ err error }

func (e *sinkError) Error() string { return e.err.Error() }
func (e *sinkError) Unwrap() error { return e.err }

// sinkFailure reports a failure of the Sink as the caller's own error, with
// ErrIntegrity as its code, as a failure of dst is in formats 1 and 2.
func sinkFailure(what string, err error) error {
	return fmt.Errorf("capsule: PAYLOAD_AGE: %s: %w: %w", what, err, datekeys.ErrIntegrity)
}

// newSecurityContext is the context of the verdicts of a capsule of format f
// whose control is c, opened at the round time unlock (spec v0.11, §29.7).
// The head digest is added when the head is read. A control that cannot be
// encoded leaves control_commit at zero: no signature verifies then, and F2
// is the verdict, though DecodeControl has already decoded it.
func newSecurityContext(c *Control, f Format, unlock time.Time, keys map[string]string) *SecurityContext {
	sc := &SecurityContext{RoundTime: unlock, AuthorKeys: keys}
	if cc, err := ControlCommit(c, f); err == nil {
		sc.ControlCommit = cc
	}
	return sc
}

// openBody runs step 17 of a format 3 capsule and step 18 (spec §63): pr is
// the plaintext of PAYLOAD_AGE, whose BODY is l bytes long and whose
// padding goes up to p. The substeps, in order:
//
//	17.2  the frame of BODY and the area (§29.2), ErrIntegrity;
//	17.3  security, layers 2 and 3 (§29.3), without a code;
//	17.4  the head, layers 2 to 4 (§29.4 to §29.6);
//	17.5  the files fill CONTENT, ErrIntegrity;
//	17.6  the verdicts of security (§29.7), without a code;
//	17.7  the SHA-256 of each file, ErrIntegrity;
//	17.8  the padding, ErrIntegrity.
//
// A failure of age after its header, or a plaintext whose length is not P,
// is ErrIntegrity and prevails; otherwise the first substep that fails
// decides. openBody stops early only with ErrIntegrity when that is already
// the final code: on a failure of age, or of a substep of ErrIntegrity with
// no earlier failure of another code. After a failure of another code, at
// 17.4, it reads PAYLOAD_AGE to EOF before reporting it.
func openBody(pr io.Reader, l, p uint64, sink Sink, reg extension.Registry, sc *SecurityContext, accept func(Verdicts) error, out *Opened) (err error) {
	r := &plainReader{r: pr}
	begun := false
	defer func() {
		if err != nil && begun {
			sink.Abort()
		}
	}()

	// 17.2: the frame of BODY and the area.
	b, err := readN(r, BodyFrameSize, p)
	if err != nil {
		return err
	}
	frame, err := ParseBodyFrame(b, l)
	if err != nil {
		return err
	}
	area, err := readN(r, int(frame.AreaLen), p)
	if err != nil {
		return err
	}
	if err := CheckArea(area, frame.SecurityLen); err != nil {
		return err
	}
	out.AreaLen = frame.AreaLen

	// 17.3 and 17.6: security never fails; its verdicts are shown after
	// step 18 only.
	security := area[:frame.SecurityLen]

	// 17.4: the head. Its codes other than ErrIntegrity are reported only
	// after reading to EOF.
	hb, err := readN(r, int(frame.HeadLen), p)
	if err != nil {
		return err
	}
	// The verdicts need the head digest, so they are evaluated once the head
	// bytes are read; they never fail, and no decoding of the head changes them.
	sc.HeadDigest = HeadDigest(hb)
	verdicts := EvaluateSecurityIn(security, sc)
	h, err := DecodeHead(hb, reg)
	if err != nil {
		if derr := drain(r, p); derr != nil {
			return derr
		}
		return err
	}

	// 17.5: the files fill CONTENT.
	if err := CheckHeadEnd(h, frame.ContentLength(l)); err != nil {
		return err
	}

	// 17.7: each file, to the Sink, with its SHA-256.
	if err := sink.Begin(h); err != nil {
		return sinkFailure("beginning the files", err)
	}
	begun = true
	for i := range h.Files {
		f := &h.Files[i]
		w, err := sink.Create(i)
		if err != nil {
			return sinkFailure(fmt.Sprintf("creating file %d", i+1), err)
		}
		sum, err := copyFile(w, r, f.Size, p)
		if cerr := w.Close(); err == nil && cerr != nil {
			err = &sinkError{cerr}
		}
		var se *sinkError
		switch {
		case errors.As(err, &se):
			return sinkFailure(fmt.Sprintf("writing file %d", i+1), se.err)
		case err != nil:
			return err
		case !hmac.Equal(sum, f.SHA256[:]):
			return fmt.Errorf("capsule: PAYLOAD_AGE: file %d: its SHA-256 is not the one of the head: %w", i+1, datekeys.ErrIntegrity)
		}
	}

	// 17.8: the padding, up to P, and nothing after it.
	if err := checkPadding(r, r.n, p); err != nil {
		if datekeys.Code(err) == "" {
			err = classify("PAYLOAD_AGE", ageStreamFailure, err)
		}
		return err
	}

	// The caller sees the verdicts before anything is published
	// (OpenOptions.Accept).
	if accept != nil {
		if err := accept(verdicts); err != nil {
			return &refused{err}
		}
	}

	// Step 18.
	if err := sink.Commit(); err != nil {
		return sinkFailure("committing the files", err)
	}
	out.Head, out.Verdicts = h, verdicts
	out.UnusableHeadExtensions = extension.CheckNoncriticalIn(extension.Head, h.Noncritical, reg)
	return nil
}