examples/chess-board.umsg

%unimsg 0 board-presentation/chess/v0

-- How to draw a position of @ruleset/chess/v0, in @vocab/board-presentation/v0.
--
-- THE THIRD DOCUMENT, AND THE SMALLEST. A ruleset says what is legal and never
-- what it looks like; a graphic is a resolved picture and cannot change when a
-- piece moves. What is missing between them is the function from a position to
-- a scene — and for a game played on a grid that function is not a program, it
-- is this table.
--
-- SO NOTHING HERE IS CHESS-SPECIFIC MACHINERY. A square size, two fills, a
-- glyph per piece kind, and where a baseline sits. tools/boardview reads it
-- beside the ruleset and draws any position:
--
--   (cd ../unimsg && go run ./tools/boardview -board examples/chess-board.umsg \
--      -ruleset examples/chess-ruleset.umsg -o /tmp/scene.umsg)
--
-- and the same table inverts, so a point on the picture resolves to a square
-- and a square to the moves the ruleset allows from it. Drawing and pointing
-- are one binding read in two directions.

board-presentation @board-presentation/chess/v0 {
  title    "A chess board"
  summary  "Eight files by eight ranks, dark on a1, pieces as the Unicode chess
            glyphs, and a frame carrying the file and rank names. It is a
            presentation of one ruleset and it knows none of its rules: nothing
            here mentions a legal move, a check or a capture, because none of
            those is a fact about what the board looks like."
  status   :draft
  at       2026-08-26
  presents "@ruleset/chess/v0"

  geometry {
    square 64
    margin 28
    y-axis :up
  }

  -- PARITY IS (file + rank), and a1 is file 0 rank 0, so :even is the dark
  -- square. Getting this backwards is the classic board bug and it is one row
  -- here rather than a condition somewhere in a renderer.
  squares [
    | parity  fill
    | :even   "#b58863"
    | :odd    "#f0d9b5"
  ]

  frame {
    fill   "#312a24"
    stroke "#1a1613"
    width  1
  }

  coordinates {
    font [ "Inter", :sans ]
    size 12
    fill "#e6ddd1"
  }

  -- THE GLYPHS ARE TEXT, WHICH IS A CHOICE WITH A COST. Six paths per side
  -- would be exact and would not depend on a font being present; two columns of
  -- one character each are twelve rows a person can read and correct. The cost
  -- is declared rather than hidden: :font names the stack, and a host without
  -- any of them draws the wrong thing.
  glyphs [
    | piece    white  black
    | :king    "♔"    "♚"
    | :queen   "♕"    "♛"
    | :rook    "♖"    "♜"
    | :bishop  "♗"    "♝"
    | :knight  "♘"    "♞"
    | :pawn    "♙"    "♟"
  ]

  glyph-style {
    font     [ "Segoe UI Symbol", "Noto Sans Symbols 2", "DejaVu Sans", :sans ]
    size     46
    fill     "#141414"
    baseline 16

    -- A PIECE ON A MARKED SQUARE IS DRAWN LARGER, which is how a person sees
    -- that the piece they touched is the one that will move. Fifteen percent is
    -- enough to read at a glance and small enough that the glyph stays inside
    -- its cell -- 46 becomes 53, against a square of 64.
    emphasis 115
  }

  -- WHERE THE PIECES THAT ARE NO LONGER ON THE BOARD GO. A band above and
  -- below, so the picture is the board plus what it has cost. The size is well
  -- under a square's: a tray is read as a count and never pointed at, and a
  -- captured piece drawn at playing size reads as a piece that could move.
  tray {
    height   40
    size     26
    spacing  24
    baseline 9
    -- The light square's colour. A captured piece is drawn in :glyph-style's
    -- near-black, as a piece on the board is, so the band it stands on must be
    -- the ground that colour was chosen against -- see
    -- a-tray-glyph-keeps-its-own-colour.
    ground   "#f0d9b5"
  }

  -- HOW A FINISHED GAME IS DECLARED. Over the board rather than beside it,
  -- because the board is what a person is looking at when it happens, and
  -- translucent so the position that produced the result is still readable
  -- underneath.
  outcome {
    height   84
    size     38
    ground   "#1a1613"
    opacity  0.82
    fill     "#f0d9b5"
    baseline 13
    -- THE SIDE NAME IS THE RULESET'S AND ARRIVES LOWERCASE, so the wording
    -- puts it where a lowercase word belongs. Capitalising it would be a rule
    -- about a language, and the module that substitutes it knows none.
    won      "winner: {side}"
    drawn    "drawn — no winner"
  }

  -- WHAT A HIGHLIGHT LOOKS LIKE, and the reason a presentation has any say in
  -- it: the squares that can be highlighted come from the RULESET — the moves
  -- legal from the square a person touched — and only how they are drawn is a
  -- presentation decision. The list is computed; the colour is declared.
  marks [
    | mark       fill       opacity
    | :selected  "#f4f169"  0.55
    | :move      "#6f9f4f"  0.55
    | :capture   "#c05a3e"  0.6
    | :last      "#c9c26a"  0.45
  ]
}