git.giacomolli.uk - onechat/blob - view.go

   1 package main
   2
   3 import (
   4         "fmt"
   5         "math"
   6         "math/bits"
   7         "strconv"
   8         "strings"
   9 )
  10
  11 const (
  12         // viewChatHigh and viewChatLow are the limits of the chat view in
  13         // bytes.
  14         viewChatHigh = 128000
  15         viewChatLow  = 64000
  16
  17         // viewCompactionHigh and viewCompactionLow are the limits of the
  18         // compaction view in bytes.
  19         viewCompactionHigh = 32000
  20         viewCompactionLow  = 16000
  21
  22         // viewPending is the text of a line that is not built yet.
  23         viewPending = "(not summarized yet: zoom it)"
  24 )
  25
  26 // newView returns an empty view with the limits high and low.
  27 func newView(high, low int) *View {
  28         return &View{
  29                 high:  high,
  30                 low:   low,
  31                 nodes: make(map[NodeKey]Node),
  32         }
  33 }
  34
  35 // restoreView returns the view of state with the limits high and low. Each line
  36 // must continue the one before it, and each line above level 0 must be built.
  37 func restoreView(state ViewState, high, low int, tree ViewNodeReader) (*View, error) {
  38         v := newView(high, low)
  39         v.merging = state.Merging
  40         for _, pair := range state.Lines {
  41                 line := NodeKey{pair[0], pair[1]}
  42                 if line.level < 0 || line.level >= bits.UintSize-2 || line.index < 0 || line.index<<line.level != v.end {
  43                         return nil, fmt.Errorf("view line (%d, %d) does not start at message %d", line.level, line.index, v.end)
  44                 }
  45
  46                 built, err := v.load(tree, line)
  47                 if err != nil {
  48                         return nil, err
  49                 }
  50
  51                 if line.level > 0 && !built {
  52                         return nil, fmt.Errorf("view line (%d, %d) is not built", line.level, line.index)
  53                 }
  54
  55                 v.lines = append(v.lines, line)
  56                 v.end = nodeEnd(line.level, line.index)
  57         }
  58
  59         return v, nil
  60 }
  61
  62 // View is a list of tree nodes that covers messages 0 to end-1 without a gap,
  63 // oldest first. Each new message appends its leaf as a line. Once the size of
  64 // the view goes above high, one batch merges the most due sibling pairs until
  65 // the size is at most low. A batch merges only pairs whose parent is built. If
  66 // the batch cannot get to low, it continues at each new message until it does.
  67 //
  68 // The view keeps the nodes of its built lines in memory, so its size and its
  69 // text need no read of the tree. The size of a line is the size of its node
  70 // text, or the size of the pending text when the node is not built.
  71 //
  72 // For the sibling pair (l, i) and (l, i+1), due is (T - last) / 2^l, with T the
  73 // number of messages and last the ID of the last message of the pair. That is
  74 // how long ago the pair ended, measured in its own line size. With a budget of
  75 // the length of the rollback push list of the reference design, this order
  76 // makes the same merges as the push. The oldest pair wins a tie.
  77 type View struct {
  78         high    int
  79         low     int
  80         lines   []NodeKey
  81         nodes   map[NodeKey]Node
  82         end     int
  83         merging bool
  84 }
  85
  86 // State returns the saved form of the view.
  87 func (v *View) State() ViewState {
  88         state := ViewState{
  89                 Lines:   make([][2]int, len(v.lines)),
  90                 Merging: v.merging,
  91         }
  92
  93         for n, line := range v.lines {
  94                 state.Lines[n] = [2]int{line.level, line.index}
  95         }
  96
  97         return state
  98 }
  99
 100 // Append adds the leaf of the next message as a line and merges when the view
 101 // is too large. It tells if a merge changed the view.
 102 func (v *View) Append(tree ViewNodeReader) (bool, error) {
 103         line := NodeKey{0, v.end}
 104         _, err := v.load(tree, line)
 105         if err != nil {
 106                 return false, err
 107         }
 108
 109         v.lines = append(v.lines, line)
 110         v.end++
 111
 112         return v.fit(tree)
 113 }
 114
 115 // Coarsen takes each merge of other, a view of the same messages, and then
 116 // merges in one batch until the size is at most low. Each line of the result is
 117 // the coarser of the two lines that start at its first message. Dyadic ranges
 118 // either nest or do not overlap, so the result covers the messages without a
 119 // gap.
 120 func (v *View) Coarsen(other *View, tree ViewNodeReader) error {
 121         lines := make([]NodeKey, 0, len(v.lines))
 122         nodes := make(map[NodeKey]Node, len(v.nodes))
 123         mine, theirs := 0, 0
 124         for start := 0; start < v.end; {
 125                 line := v.lines[mine]
 126                 node, built := v.nodes[line]
 127                 if other.lines[theirs].level > line.level {
 128                         line = other.lines[theirs]
 129                         node, built = other.nodes[line]
 130                 }
 131
 132                 lines = append(lines, line)
 133                 if built {
 134                         nodes[line] = node
 135                 }
 136
 137                 start = nodeEnd(line.level, line.index)
 138                 for mine < len(v.lines) && v.lines[mine].index<<v.lines[mine].level < start {
 139                         mine++
 140                 }
 141
 142                 for theirs < len(other.lines) && other.lines[theirs].index<<other.lines[theirs].level < start {
 143                         theirs++
 144                 }
 145         }
 146
 147         v.lines = lines
 148         v.nodes = nodes
 149         v.merging = true
 150         _, err := v.fit(tree)
 151
 152         return err
 153 }
 154
 155 // Refresh keeps node when it is the node of a pending line.
 156 func (v *View) Refresh(node Node) {
 157         key := NodeKey{node.Level, node.Index}
 158         for n := len(v.lines) - 1; n >= 0; n-- {
 159                 line := v.lines[n]
 160                 if line == key {
 161                         v.nodes[key] = node
 162                         return
 163                 }
 164
 165                 if line.index<<line.level <= key.index<<key.level {
 166                         return
 167                 }
 168         }
 169 }
 170
 171 // Render returns the view as the model reads it: one line "id+n|text" for each
 172 // node inside <chat> tags, with the newlines of the text as spaces.
 173 func (v *View) Render() string {
 174         return "<chat>\n" + strings.Join(v.LinesWithout(nil), "") + "</chat>"
 175 }
 176
 177 // LinesWithout returns the lines of the view as the model reads them, each with
 178 // its newline, without the leaf lines of the messages in skip. A line that
 179 // merges such a message with others stays.
 180 func (v *View) LinesWithout(skip map[int]bool) []string {
 181         lines := make([]string, 0, len(v.lines))
 182         for _, line := range v.lines {
 183                 if line.level == 0 && skip[line.index] {
 184                         continue
 185                 }
 186
 187                 var text strings.Builder
 188                 v.renderLine(&text, line)
 189                 lines = append(lines, text.String())
 190         }
 191
 192         return lines
 193 }
 194
 195 // Context returns the lines of the view before message end as the model reads
 196 // them. It stops at the first line that is not built, so a compaction never
 197 // sees a pending mark or a part of a message.
 198 func (v *View) Context(end int) string {
 199         var text strings.Builder
 200         text.WriteString("<chat>\n")
 201         for _, line := range v.lines {
 202                 _, built := v.nodes[line]
 203                 if !built || nodeEnd(line.level, line.index) > end {
 204                         break
 205                 }
 206
 207                 v.renderLine(&text, line)
 208         }
 209
 210         text.WriteString("</chat>")
 211
 212         return text.String()
 213 }
 214
 215 // load keeps the node of line when it is built, and tells if it is.
 216 func (v *View) load(tree ViewNodeReader, line NodeKey) (bool, error) {
 217         node, built, err := tree.Node(line.level, line.index)
 218         if err != nil || !built {
 219                 return false, err
 220         }
 221
 222         v.nodes[line] = node
 223
 224         return true, nil
 225 }
 226
 227 // fit merges the most due pairs while a batch runs, and tells if a merge
 228 // changed the view.
 229 func (v *View) fit(tree ViewNodeReader) (bool, error) {
 230         size := v.Size()
 231         if size > v.high {
 232                 v.merging = true
 233         }
 234
 235         merged := false
 236         for v.merging && size > v.low {
 237                 best, err := v.mostDue(tree)
 238                 if err != nil || best < 0 {
 239                         return merged, err
 240                 }
 241
 242                 left, right := v.lines[best], v.lines[best+1]
 243                 parent := NodeKey{left.level + 1, left.index / 2}
 244                 _, err = v.load(tree, parent)
 245                 if err != nil {
 246                         return merged, err
 247                 }
 248
 249                 size += v.lineSize(parent) - v.lineSize(left) - v.lineSize(right)
 250                 delete(v.nodes, left)
 251                 delete(v.nodes, right)
 252                 v.lines[best] = parent
 253                 v.lines = append(v.lines[:best+1], v.lines[best+2:]...)
 254                 merged = true
 255         }
 256
 257         v.merging = false
 258
 259         return merged, nil
 260 }
 261
 262 // renderLine writes one view line, "id+n|text", and a newline.
 263 func (v *View) renderLine(text *strings.Builder, line NodeKey) {
 264         body := viewPending
 265         node, built := v.nodes[line]
 266         if built {
 267                 body = strings.ReplaceAll(node.Text, "\n", " ")
 268         }
 269
 270         text.WriteString(strconv.Itoa(line.index << line.level))
 271         text.WriteByte('+')
 272         text.WriteString(strconv.Itoa(1 << line.level))
 273         text.WriteByte('|')
 274         text.WriteString(body)
 275         text.WriteByte('\n')
 276 }
 277
 278 // Size returns the sum of the line sizes.
 279 func (v *View) Size() int {
 280         size := 0
 281         for _, line := range v.lines {
 282                 size += v.lineSize(line)
 283         }
 284
 285         return size
 286 }
 287
 288 // mostDue returns the position of the left line of the most due sibling pair
 289 // whose parent is built, or -1 when there is no such pair.
 290 func (v *View) mostDue(tree ViewNodeReader) (int, error) {
 291         best := -1
 292         bestDue := 0.0
 293         for n := 0; n+1 < len(v.lines); n++ {
 294                 left, right := v.lines[n], v.lines[n+1]
 295                 if left.index%2 != 0 || right.level != left.level || right.index != left.index+1 {
 296                         continue
 297                 }
 298
 299                 last := nodeEnd(right.level, right.index) - 1
 300                 due := math.Ldexp(float64(v.end-last), -left.level)
 301                 if best >= 0 && due <= bestDue {
 302                         continue
 303                 }
 304
 305                 built, err := tree.Built(left.level+1, left.index/2)
 306                 if err != nil {
 307                         return -1, err
 308                 }
 309
 310                 if built {
 311                         best = n
 312                         bestDue = due
 313                 }
 314         }
 315
 316         return best, nil
 317 }
 318
 319 // lineSize returns the size of a view line.
 320 func (v *View) lineSize(line NodeKey) int {
 321         node, built := v.nodes[line]
 322         if !built {
 323                 return len(viewPending)
 324         }
 325
 326         return node.Size
 327 }
 328
 329 // ViewState is the saved form of a View. The limits are not part of it.
 330 type ViewState struct {
 331         Lines   [][2]int `json:"lines"`
 332         Merging bool     `json:"merging"`
 333 }
 334
 335 // ViewNodeReader gives the built nodes of a tree.
 336 type ViewNodeReader interface {
 337         Built(level, index int) (bool, error)
 338         Node(level, index int) (Node, bool, error)
 339 }