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package main

// Graph rendering: Instance -> DOT -> SVG via graphviz.
//
// Server-side graphviz rather than an in-browser layout library: dot's layered
// layout is what produces the look of the Practical Alloy figures, instances are
// small enough that layout cost is irrelevant, and it keeps the browser a dumb
// viewer with no JS build step.

import (
	"bytes"
	"context"
	"fmt"
	"os/exec"
	"sort"
	"strings"
	"time"
)

// edgePalette is Alloy's CLASSIC palette (DotColor MAGIC resolution order),
// assigned to relations in declaration order so each relation gets its own
// colour, as in the GUI.
var edgePalette = []string{
	"#e41a1c", // red
	"#a65628", // brown
	"#ff7f00", // orange
	"#4daf4a", // green
	"#377eb8", // blue
	"#984ea3", // purple
}

// RenderDOT converts one instance to DOT source.
func RenderDOT(inst *Instance, theme *Theme) string {
	if theme == nil {
		theme = DefaultTheme()
	}

	// Relations marked attribute="yes" are folded into node labels instead of
	// being drawn. Do this first so nodes know their attributes before emission.
	//
	// Reset any attributes from a previous render; instances are re-rendered
	// whenever the theme changes.
	for _, a := range inst.Atoms {
		a.Attrs = nil
	}
	// Grouped per relation per node, so a node with (A,B) and (A,C) in F gets a
	// single "F: B, C" line rather than two lines. Matches
	// StaticGraphMaker.edgesAsAttribute.
	for _, rel := range inst.Relations {
		if !theme.edgeAttribute(rel.Name) {
			continue
		}
		if rel.IsSkolem && theme.HideSkolem {
			continue
		}
		perNode := map[*Atom][]string{}
		order := []*Atom{}
		for _, t := range rel.Tuples {
			if len(t) < 2 {
				continue
			}
			src, ok := inst.AtomByName[t[0]]
			if !ok {
				continue
			}
			// Arity > 2 renders the remaining columns joined by "->".
			parts := make([]string, 0, len(t)-1)
			for _, name := range t[1:] {
				if at, ok := inst.AtomByName[name]; ok {
					parts = append(parts, at.Label())
				} else {
					parts = append(parts, name)
				}
			}
			if _, seen := perNode[src]; !seen {
				order = append(order, src)
			}
			perNode[src] = append(perNode[src], strings.Join(parts, "->"))
		}
		for _, src := range order {
			src.Attrs = append(src.Attrs, rel.Name+": "+strings.Join(perNode[src], ", "))
		}
	}

	// Decide which atoms get nodes. An atom is drawn unless its type is hidden
	// by the theme.
	drawn := make(map[string]bool, len(inst.Atoms))
	for _, a := range inst.Atoms {
		if theme.nodeVisible(a.TypeChain) {
			drawn[a.Name] = true
		}
	}

	var b strings.Builder
	b.WriteString("digraph alloy {\n")
	b.WriteString("  graph [rankdir=TB, nodesep=0.35, ranksep=0.5, bgcolor=\"transparent\"];\n")
	b.WriteString("  node  [fontname=\"Helvetica\", fontsize=11, style=filled, penwidth=1];\n")
	b.WriteString("  edge  [fontname=\"Helvetica\", fontsize=10];\n\n")

	// Nodes, in a stable order so the SVG does not churn between renders.
	atoms := append([]*Atom(nil), inst.Atoms...)
	sort.Slice(atoms, func(i, j int) bool {
		if atoms[i].Type != atoms[j].Type {
			return atoms[i].Type < atoms[j].Type
		}
		return atoms[i].Index < atoms[j].Index
	})
	for _, a := range atoms {
		if !drawn[a.Name] {
			continue
		}
		fmt.Fprintf(&b, "  %s [label=%s, shape=%s, fillcolor=%q];\n",
			dotID(a.Name), dotString(a.FullLabel(theme.HideSkolem)),
			theme.nodeShape(a.TypeChain), theme.nodeColor(a.TypeChain))
	}
	b.WriteString("\n")

	// Edges.
	ci := 0
	for _, rel := range inst.Relations {
		if !theme.edgeVisible(rel.Name) || theme.edgeAttribute(rel.Name) {
			continue
		}
		if rel.IsSkolem && theme.HideSkolem {
			continue
		}
		color := edgePalette[ci%len(edgePalette)]
		ci++
		for _, t := range rel.Tuples {
			if len(t) < 2 {
				continue
			}
			src, dst := t[0], t[len(t)-1]
			if !drawn[src] || !drawn[dst] {
				continue
			}
			// Arity > 2: draw first -> last and put the intermediate atoms in
			// the label, matching the GUI ("R [B, C]" for tuple (A,B,C,D)).
			label := rel.Name
			if len(t) > 2 {
				mids := make([]string, 0, len(t)-2)
				for _, m := range t[1 : len(t)-1] {
					if at, ok := inst.AtomByName[m]; ok {
						mids = append(mids, at.Label())
					} else {
						mids = append(mids, m)
					}
				}
				label = fmt.Sprintf("%s [%s]", rel.Name, strings.Join(mids, ", "))
			}
			fmt.Fprintf(&b, "  %s -> %s [label=%s, color=%q, fontcolor=%q];\n",
				dotID(src), dotID(dst), dotString(label), color, color)
		}
	}

	b.WriteString("}\n")
	return b.String()
}

// dotID quotes an atom name for use as a DOT node id. Atom labels contain '$',
// which DOT does not accept unquoted.
func dotID(s string) string { return dotString(s) }

// dotString renders a Go string as a DOT quoted string, escaping quotes,
// backslashes and newlines (\n is meaningful to DOT as a line break).
func dotString(s string) string {
	var b strings.Builder
	b.WriteByte('"')
	for _, r := range s {
		switch r {
		case '"':
			b.WriteString("\\\"")
		case '\\':
			b.WriteString("\\\\")
		case '\n':
			b.WriteString("\\n")
		default:
			b.WriteRune(r)
		}
	}
	b.WriteByte('"')
	return b.String()
}

// RenderSVG pipes DOT source through graphviz.
//
// The timeout guards against a pathological layout wedging the UI; graphviz on
// instances this small is a few milliseconds.
func RenderSVG(ctx context.Context, dot string) (string, error) {
	ctx, cancel := context.WithTimeout(ctx, 20*time.Second)
	defer cancel()

	cmd := exec.CommandContext(ctx, "dot", "-Tsvg")
	cmd.Stdin = strings.NewReader(dot)
	var out, errBuf bytes.Buffer
	cmd.Stdout = &out
	cmd.Stderr = &errBuf
	if err := cmd.Run(); err != nil {
		return "", fmt.Errorf("dot -Tsvg: %w: %s", err, strings.TrimSpace(errBuf.String()))
	}
	return stripXMLPreamble(out.String()), nil
}

// stripXMLPreamble removes the XML declaration and DOCTYPE that graphviz emits,
// so the SVG can be inlined directly into an HTML document.
func stripXMLPreamble(svg string) string {
	if i := strings.Index(svg, "<svg"); i > 0 {
		return svg[i:]
	}
	return svg
}