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119 lines
2.8 KiB
119 lines
2.8 KiB
// Package constraint implements an extremely simple constraint engine.
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// Elements, and constraints on those elements, are given to the engine, which
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// uses those constraints to generate an output. Elements are defined as a
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// string
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package constraint
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import (
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"github.com/mediocregopher/ginger/gg"
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)
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// Constraint describes a constraint on an element. The Elem field must be
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// filled in, as well as exactly one other field
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type Constraint struct {
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Elem string
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// LT says that Elem is less than this element
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LT string
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}
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var ltEdge = gg.NewValue("lt")
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// Engine processes sets of constraints to generate an output
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type Engine struct {
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g *gg.Graph
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vals map[string]gg.Value
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}
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// NewEngine initializes and returns an empty Engine
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func NewEngine() *Engine {
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return &Engine{g: gg.ZeroGraph, vals: map[string]gg.Value{}}
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}
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func (e *Engine) getVal(elem string) gg.Value {
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if val, ok := e.vals[elem]; ok {
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return val
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}
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val := gg.NewValue(elem)
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e.vals[elem] = val
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return val
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}
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// AddConstraint adds the given constraint to the engine's set, returns false if
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// the constraint couldn't be added due to a conflict with a previous constraint
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func (e *Engine) AddConstraint(c Constraint) bool {
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elem := e.getVal(c.Elem)
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g := e.g.AddValueIn(gg.ValueOut(elem, ltEdge), e.getVal(c.LT))
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// Check for loops in g starting at c.Elem, bail if there are any
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{
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seen := map[*gg.Vertex]bool{}
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start := g.ValueVertex(elem)
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var hasLoop func(v *gg.Vertex) bool
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hasLoop = func(v *gg.Vertex) bool {
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if seen[v] {
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return v == start
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}
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seen[v] = true
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for _, out := range v.Out {
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if hasLoop(out.To) {
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return true
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}
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}
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return false
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}
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if hasLoop(start) {
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return false
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}
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}
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e.g = g
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return true
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}
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// Solve uses the constraints which have been added to the engine to give a
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// possible solution. The given element is one which has been added to the
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// engine and whose value is known to be zero.
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func (e *Engine) Solve() map[string]int {
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m := map[string]int{}
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if len(e.g.ValueVertices()) == 0 {
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return m
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}
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vElem := func(v *gg.Vertex) string {
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return v.Value.V.(string)
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}
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// first the roots are determined to be the elements with no In edges, which
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// _must_ exist since the graph presumably has no loops
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var roots []*gg.Vertex
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e.g.Iter(func(v *gg.Vertex) bool {
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if len(v.In) == 0 {
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roots = append(roots, v)
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m[vElem(v)] = 0
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}
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return true
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})
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// sanity check
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if len(roots) == 0 {
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panic("no roots found in graph somehow")
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}
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// a vertex's value is then the length of the longest path from it to one of
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// the roots
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var walk func(*gg.Vertex, int)
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walk = func(v *gg.Vertex, val int) {
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if elem := vElem(v); val > m[elem] {
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m[elem] = val
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}
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for _, out := range v.Out {
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walk(out.To, val+1)
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}
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}
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for _, root := range roots {
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walk(root, 0)
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}
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return m
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}
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