# The Markdown function is a markdown to html converter for a "good
# enough" subset of Github-flavored markdown, as specified at
# https://github.github.com/gfm/ .

# Extensions are used whenever a source filename is given (mainly just
# so testing won't use them). They:
# - Add id= slugs to headers that match Githubs, for linking
# - Adjust relative links to account for filename changes
# - Highlight inline and block code as BQN
# - Place code blocks in <pre> tags only, not <pre><code>
# - Insert results into doubly-indented (8 spaces) code blocks
# - Add links to open and execute code in the REPL
# - Put a demo REPL in place of <!--REPL-->, for the main README

# Supports:
# - ATX headings (start with hashes #)
# - Paragraphs
# - Indented code blocks
# - Inline and raw HTML in a way that doesn't match the spec at all
# - Tables
# - Lists, unordered with single-line items only
# - Inlines: code fully, links partially, and emphasis somewhat

# Important missing features:
# - Thematic breaks like *** or ---
# - Setext headings (underlined with ==== or ----)
# - Fenced code blocks (marked off with ``` or ~~~)
# - Block quotes (start with >)
# - Strikethrough (~~text~~)
# - Images (like links)
# - Hard line breaks (trailing spaces or backslash)

# Here, a markdown file is represented as a list of its lines, which are
# strings (they don't include any line ending character).
# The html file is constructed directly as a string, using Html.

################################
# Utilities

# Linefeed
lf ← @+10

# Index of first zero, or number of leading 1s in a boolean list
Lead ← ⊑ ⊐⟜0

# 𝕨 is a list of lists. Find the first of these lists each cell of 𝕩
# belongs to.
FindGroup ← {
  i ← (∾𝕨) ⊐ 𝕩  # Index in all cells of 𝕨
  e ← +`≠¨𝕨     # Index past the end of each group of 𝕨
  e ⍋ i         # How many end-indices does each element pass?
}

# Count the number of consecutive true values up to the current element.
# To do this, subtract the index of the last false character from the
# current index.
CountRuns ← { (1+↕≠𝕩) (⊣ - ⌈`∘×) ¬𝕩 }

# 𝕩 is a string; return a mask of the characters that are escaped, that
# is, preceded by an odd number of backslashes (since a backslash can
# escape another backslash).
IsEscaped ← {
  » 2 | CountRuns 𝕩 = '\'
}

# Remove leading (∧`) and trailing (∧`⌾⌽) spaces
Trim ← { 𝕩 /˜ ¬ (∧` ∨ ∧`⌾⌽) ' '=𝕩 }

# Find whether 𝕨 was true at the last index where 𝕩 was false, in each
# position.
PrecedesGroup ← {
  # We prepend a 0 to 𝕨, so that 0 is the "before start" index, with a
  # false value, and normal indices are increased by 1.
  𝕨 ∾˜↩ 0
  inds ← 1 + ↕≠𝕩
  # Zero out indices where 𝕩 was true, and find the greatest index so
  # far at each position.
  last ← ⌈` inds × ¬𝕩
  last ⊏ 𝕨
}

# 𝕨 is a list of possible expression start indices in any order and 𝕩 is
# the corresponding endpoints. The expressions are mutually exclusive
# and do not nest, and are enabled in index order. Return a shape ·‿2
# array where the rows give the start and end of each enabled expression
# in index order.
Trace ← {
  # 𝕨 is a list with one index for each possible start, giving a later
  # start that is known to be enabled if that one is.
  # 𝕩 is a mask of all starts known to be enabled.
  # A "stop" position that follows all expressions tells when to stop.
  # At each step the distance from a start to its successor in 𝕨 is
  # doubled, so the maximum number of steps is about 2⋆⁼≠𝕩.
  En ← {
    𝕩 ↩ 1¨⌾((𝕩/𝕨)⊸⊏)𝕩  # Starts following from an enabled one are enabled
    𝕨 ↩ ⊏˜ 𝕨           # Double the number of steps in 𝕨
    𝕨 En 𝕩             # Repeat
  }⍟{0=¯1⊑𝕩}           #        until the stop is enabled

  g ← ⍋𝕨               # Order expressions by starting index
  start ← g⊏𝕨
  end   ← g⊏𝕩
  next ← start ⍋ end   # An expression's successor starts after it ends
  next ∾↩ ≠next        # The stop node is its own successor
  enabled ← ¯1 ↓ next En (≠next)↑1  # Search and remove the stop
  enabled / start≍˘end # List of enabled starts and ends
}

# Join lines with newline characters. Include a trailing newline.
JoinLines ← ∾ ∾⟜lf¨

# Given a list of begin-end pairs run together, return a list
Ranges ← {
  R ← {𝕨+↕𝕩¬𝕨}  # Single range
  𝕩 ↩ ∘‿2 ⥊ 𝕩   # Reshape into pairs
  ∾ R¨˝˘ 𝕩
}
# ∊⟜Ranges assuming 𝕨 is sorted
InRanges ← {
  𝕩 +↩ 2|↕≠𝕩  # Since ⍋ works with half-open intervals
  2 | 𝕩 ⍋ 𝕨
}

# Create an html node from a tag name and interior text.
Html ← {open 𝕊 contents:
  close ← (⊑open⊐" ") ↑ open
  ∾ ⟨"<",open,">" , contents , "</",close,">"⟩
}

# Insert and remove things from the list 𝕩:
# - include is the mask of elements to keep in 𝕩
# - add is a list of lists to be inserted
# - pos is the list of positions where they should start
# Elements are added just after the given position in 𝕩, in the order
# they appear in ∾add.
Modify ← { ⟨include,add,pos⟩𝕊𝕩:
  ((/include)∾(≠¨add)/pos) ⍋⊸⊏ (include/𝕩)∾∾add
}

# URL encoding for links to the REPL
UTF32 ← {
  p ← 2⋆10
  s ← p×52+2+↕3  # Surrogate character boundaries
  h ← 1 = c ← (≠s)|s⍋𝕩
  h ¬⊸/ (»h×p×(2⋆6)+⊢)⊸+ 𝕩 - c⊏0∾s
}⍟(2=≠"𝕩")∘-⟜@
UTF8 ← ∾ (2⋆7) (⊣+(2⋆6){𝕨 ≤◶⟨⥊⊢-2×-⟜𝕗 ⋄ 𝕗(|∾˜(2÷˜⌊⟜𝕨)𝕊⌊∘÷˜)⊢⟩ 𝕩}¨) UTF32
Base64 ← {
  b64 ← Ranges "AZaz09++//"
  b←3|↕l←≠u←UTF8 𝕩
  M←((0<↕4)⥊˜≠)⊸× (1+0=b)⊸/
  v←(4⋆1+b) ((⌊∘÷˜) «⊸+○M (64÷⊣)×|) u
  (v⊏b64)∾(3|-l)⥊'='
}

# HTML and JS for demo REPL used in main README
repl ← 1↓"
<div class='cont'>
  <div class='kb'></div>
  <div class='rel'>
    <textarea class='code' rows='1' autofocus><⟜'a'⊸/ ""Big Questions Notation""</textarea>
    <svg class='demo' viewBox='0 -6 4 12'><path d='M1 -6H0L1 0L0 6H1L4 0z'/></svg>
  </div>
  <pre class='rslt'>""B Q N""</pre>
</div>"
loadRepl ← "<script src='bqn.js'></script><script src='repl.js'></script>"

# Various URLs
siteURL ← "https://mlochbaum.github.io/BQN/"
tryURL  ← siteURL∾"try.html#code="
repoURL ← "https://github.com/mlochbaum/BQN"
blobURL ← repoURL∾"/blob/master/"

# Environments
_getCodeExec ← {𝕗⋄⍎}
_getSvgExec ← {𝕗⋄⟨•path∾"svg.bqn","⍎"⟩ •EX •path∾"dzref"}


################################
Markdown ← {filename𝕊𝕩:
  extensions ← filename ≢ 0
  path ← extensions◶""‿(⊢/˜·∨`⌾⌽'/'⊸=) filename

  CodeExec ← @_getCodeExec
  SvgExec ← @_getSvgExec

  ######
  # First we classify each line based on the type of block it can start.
  ClassifyLine ← (0<≠)◶(0‿0)‿{
    ind ← ⊑ lineChars FindGroup ⊏𝕩
    getLen ← ind ⊑ lineClas∾⟨0˜⟩
    l ← GetLen 𝕩
    ⟨ind ∧ l>0 ⋄ l⟩
  }

  # Character entity escaping
  # In order to use this with other modifications such as highlighting,
  # CharEntities returns a mask of characters to be escaped, and their
  # corresponding escapes.
  CharEntities ← {1¨⊸𝕊𝕩;  # 𝕨 gives characters to potentially escape
    # The string gives escapes and their names, separated by spaces.
    # First split it on the first character.
    ce ← (1-˜¬×+`)∘=⟜⊑⊸⊔ " ""quot &amp <lt >gt"
    # Characters to escape are given first
    chars ← ⊑¨ce
    # HTML character entities start with & and end with ;
    entities ← ("&"∾∾⟜";")¨ 1↓¨ce

    # Replace a character if 𝕨 is not set and it's on our list.
    ind ← chars ⊐ 𝕩
    useEntity ← 𝕨 ∧ ind < ≠chars
    ⟨¬ useEntity , entities ⊏˜ useEntity/ind , /useEntity⟩
  }

  # Non-empty lines in code blocks have 4 leading spaces
  ProcCode ← {
    # Strip the leading spaces
    𝕩 ↩ 4 ↓¨ 𝕩
    code ← JoinLines 𝕩

    # Highlight and unescape html-unsafe characters
    c‿ci ← extensions◶(2⥊<⟨⟩)‿GetHighlights code
    em‿e‿ei ← CharEntities code

    # If every line is indented by at least 4 additional spaces, we will
    # execute each one and insert the results.
    addRslt ← ∧´ ' ' = ∾ 4 (⌊⟜≠ ↑ ⊢)¨ 𝕩
    # Don't show assignment results by default
    ShowRslt ← {
      depth ← +` "(){}⟨⟩" (⊣(≠⊸>ׯ1⋆2|⊢)⊐) 𝕩
      𝕩 /˜↩ ¬ ∨`⌾⌽ (0=depth) ∧ 𝕩∊"⋄,"  # Just the last statement
      g ← 𝕩∊"←↩"
      (⊑g⊐1) (<⟜(≠g))◶⟨1,¬(" "∾∾idChars)∧´∘∊˜↑⟩ 𝕩
    }
    r‿ri ← addRslt◶(2⥊<⟨⟩)‿{
      ⟨ (ShowRslt ⊣◶⟨"",(⥊∾⟜lf⎉1)∘Fmt∘⊢⟩ CodeExec)⎊"ERROR"⍟(0<≠)¨ 𝕩
        1 -˜ +` 1 + ≠¨ 𝕩   ⟩ # Don't forget the trailing newline
    } 𝕩

    Link ← {
      c ← tryURL ∾ "&run" ∾˜ Base64 ¯1 ↓ JoinLines 4↓¨𝕩
      ("a class=""replLink"" title=""Open in the REPL"" target=""_blank"" href="∾""""(∾∾⊣)c) Html "↗️"
    }
    mod ← ⟨em,e∾c∾r,ei∾ci∾ri⟩ Modify code
    𝕩 Link⊸∾⍟addRslt "pre" Html "code" Html⍟(¬extensions) mod
  }

  # Headings start with #, and require 1-6 #s followed by a space.
  # Any trailing #s are ignored.
  LenHeading ← {
    n ← Lead 𝕩='#'
    l ← (0<n) ∧ (6≥n)
    s ← n (<⟜≠)◶⟨1,' '=⊑⟩ 𝕩 # Character after hashes must be a space, if any
    n × l ∧ s
  }
  ProcHeading ← {
    tag ← "h" ∾ '0'+𝕨       # h3 for 3 hashes, etc.
    𝕩 ↓˜↩ 𝕨+1
    trsp ← ∧`⌾⌽ 𝕩=' '
    tail ← ∧`⌾⌽ trsp∨𝕩='#'  # Mask of trailing hashes
    f ← <⟜« tail            # Character before trailing hashes
    𝕩 /˜↩ ¬ f (⊑⟨"\"," ",""⟩⊐<f/𝕩)◶⟨⊣,⊢,⊢,0¨⊢⟩ tail
    # Add an id, containing only a-z, digits, and hyphens
    Slugify ← {
      𝕩 ↩ '-'¨⌾((𝕩=' ')⊸/) 𝕩  # Replace spaces with dashes
      bounds ← ⥊ "Aa"+⌜0‿26   # Of the upper and lowercase alphabet
      # Lowercase alphabetic characters and remove special characters
      b ← bounds ⍋ 𝕩
      ((2|b)∨∊⟜("-"∾'0'+↕10))⊸/ 𝕩+32×1=b
    }
    extensions { tag ∾↩ " id="∾""""(∾∾⊣) Slugify 𝕩 }⍟⊣ 𝕩
    tag Html ProcInline Trim 𝕩
  }⟜⊑

  # List items start with a bullet (unordered) or number (ordered).
  LenBullet ← +⟜× ·≤⟜4⊸× ·Lead ' '=1⊸↓
  ProcBullet ← {
    "ul" Html lf ∾ JoinLines ("li" Html ProcInline)¨ 𝕨 ↓¨ 𝕩
  }
  LenListNum ← { # Not used yet
    n ← Lead 1="0:"⍋𝕩
    l ← (1≤n) ∧ (9≥n)
    ' ' = n ↓ 𝕩
    t ← n↓(n+2)↑𝕩
    l ∧ (" " ≡ 1↓t) ∧ ⊑(")." ∊˜ 1↑t)
  }

  # Table detection handled specially because the spec is... special
  CutTableRow ← {
    b ← '|' = 𝕩            # Mask of bars
    o ← (¬b) ≠○Lead ' '=𝕩  # Leading | omitted
    r ← b > » '\' = 𝕩      # Non-escaped bars
    1 -˜ (¬r∨«b>r) × o + +` r
  }
  alignments ← (" align="""∾∾⟜"""")⍟(0<≠)¨ ""‿"right"‿"left"‿"center"
  ProcTable ← {
    rows ← (Trim¨ CutTableRow⊸⊔)¨ 𝕩
    incl ← ¬ rule ← (∧´∾∊"-:"˙)¨ rows
    align ← alignments ⊏˜ (+˜⊸+´0‿¯1⊏⊢)¨ ':' = ⊑ rule / rows
    rows ↩ (((≠align)⌊≠)⊸↑ ProcInline¨)¨⌾(incl⊸/) rows
    rows ↩ (⊏rows) (⊢ ∾ ⟨""⟩ /˜ 0⌈-○≠)¨ rows
    rowType ← incl / +` rule  # Head or body
    tags ← rowType ⊏ "th"‿"td"
    DoRow ← { lf ∾ JoinLines 𝕨 Html¨ 𝕩 }
    rows ↩ (<˘ tags ∾⌜ align) DoRow¨ incl/rows
    rowGroups ← ¯1 ↓ rowType ⊔○(∾⟜2) "tr"⊸Html¨ rows
    sections ← "thead"‿"tbody" Html⟜(lf ∾ JoinLines)¨ rowGroups
    "table" Html lf ∾ JoinLines (0 < ≠¨rowGroups) / sections
  }

  # Paragraphs
  ProcParagraph ← {
    "p" Html ProcInline ¯1 ↓ JoinLines Trim⌾(¯1⊸⊑) (Lead ' '⊸=)⊸↓¨ 𝕩
  }

  # HTML blocks
  # Lazy rule: if it starts with < and contains >, it's probably HTML
  IsHtmlBlock ← ("<!--"≡4↑⊢)◶('>'∨´∘=⊢)‿2
  useRepl←0 ⋄ GetRepl ← {𝕩⋄ useRepl↩1 ⋄ repl}
  ProcComment ← ⟨"<!--REPL-->"⟩⊸≢◶⟨
    GetRepl
    ("<!--SVG"≡⊑∘⊢)◶JoinLines‿(SvgExec∘JoinLines 1↓¯1↓⊢)
  ⟩
  ProcHtml ← {
    codeMask ← "<code>" (6⥊0)⊸»⊸(>○(⌈`(1+↕∘≠)⊸×))○(⍷⟜𝕩 ∾ 0⥊˜1-˜≠) "</code>"
    (1¨ <⊸∾ codeMask⊸GetMultiHighlights)⊸Modify 𝕩
  }⟜JoinLines
  ProcHtmlBlock ← extensions◶JoinLines‿(<⟜2◶ProcComment‿ProcHtml)

  lineChars‿lineClas‿procFns ← <˘⍉>⟨
    ""    ‿ (!∘0)       ‿ ProcParagraph
    "#"   ‿ LenHeading  ‿ ProcHeading
    ""    ‿ 0           ‿ ProcCode
    ""    ‿ 0           ‿ ProcTable
    "-+*" ‿ LenBullet   ‿ ProcBullet
  # •d    ‿ LenListNum  ‿ ProcListNum
    "<"   ‿ IsHtmlBlock ‿ ProcHtmlBlock
  ⟩

  ######
  # Inline elements
  ProcInline ← {
    I2M ← (≠𝕩) ↑ /⁼  # Index to mask
    punc ← 𝕩 InRanges "!/:@[`{~"
    actual ← ¬ punc ∧ IsEscaped 𝕩  # backtick or *actual* backtick?

    # Code spans
    tick ← 𝕩 = '`'
    tend ← / >⟜« tick
    tcount ← CountRuns tick
    # 𝕨 are tick lengths and 𝕩 are positions, both sorted by length
    MatchTicks ← {
      # Tick runs other than the last of each length
      notLast ← =⟜« 𝕨
      # Ticks preceded by backslashes can't start code blocks, but can
      # end them. This approach is wrong for multiple ticks with a
      # leading backslash in front, which are excluded but should just
      # be treated as one shorter when leading.
      filter ← notLast / (𝕩¬𝕨) ⊏ actual
      # For leading ticks, filter by not-last; for trailing ones, rotate
      # by ¯1 to filter by not-first.
      (filter / ⌽⟜notLast / 𝕩˙)¨ 0‿¯1
    }
    tlen ← tend ⊏ tcount
    c ← Trace´ tlen MatchTicks○((⍋tlen)⊸⊏) tend
    cl ← (⊏˘c) ⊏ tcount
    ctInds ← ⥊˘ 1 + c -⌜˘ cl×⌜1‿0
    codeMask ← ≠` I2M ⥊ codeBounds ← 1‿2⊸⊏˘ ctInds
    𝕩 ↩ ' '¨⌾((codeMask∧𝕩=lf)⊸/) 𝕩
    # If span has both a leading and a trailing space, they are removed.
    remSpace ← I2M ⥊ ((1<-˜˝˘)∧·∧˝˘' '=⊏⟜𝕩)⊸/ -⟜0‿1˘ codeBounds
    codeMask ∧↩ ¬ remSpace
    ⟨code,codePos⟩ ← codeMask extensions◶(2⥊<⟨⟩)‿GetMultiHighlights 𝕩
    include ← ¬ remSpace ∨ ≠` I2M ⥊ ctInds
    codeBounds ↩ ⥊ -⟜1‿0˘ codeBounds
    unused ← actual ∧ include ∧ ¬ codeMask

    # Links
    ProcLink ← {text 𝕊 target:
      ∾⟨"<a href=""",AdjustTarget ⥊target,""">",ProcInline text,"</a>"⟩
    }
    ghPath ← blobURL∾path
    AdjustTarget ← {
      # Adjust relative *.md links, and make other relative links
      # absolute (pointing to github.com instead of github.io).
      _replaceEnd ← {old‿new‿sub‿default _r:
        (-≠old) (old≡↑)◶⟨default,sub∘↓∾new˙⟩ ⊢
      }
      RI ← "README."‿"index."‿⊢‿⊢ _replaceEnd
      R ← "md"‿"html"‿RI‿(ghPath⊸∾) _replaceEnd
      # Exclude absolute links by testing for :
      # Don't do anything to fragments (trailing #sub-heading)
      R⍟(¬𝕩∨´∘=':')⍟(0<≠)⌾((⊑𝕩⊐"#")⊸↑) 𝕩
    }⍟extensions
    # Find matched-depth [] and () pairs, then join adjacent ones
    brak ← (unused ∧ 𝕩⊸=)¨ "[]"≍"()"
    FindPairs ← { # 𝕩 is open‿close masks
      # Open and close should have the same depth, so shift close right
      depth ← +`∘-⟜»´ 𝕩
      # Get indices from masks
      ind ← ∾ inds ← /¨ 𝕩
      # Depth, index, and type for each bracket.
      # Sort by those attributes in the given order.
      # At a given depth, open and closed brackets have to alternate, so
      # now we just have to filter out the unmatched ones, that is,
      # leading close brackets and trailing open brackets at each depth.
      d‿i‿t ← <˘ ∧⌾⍉ > ⟨ind⊏depth, ind, /≠¨inds⟩
      # Preceded/followed by same-depth bracket
      p ← » f ← ∞⊸«⊸= d
      # Open brackets (0) must be followed, closed ones (1) preceded
      hasPair ← (f∧0=t) ∨ (p∧1=t)
      ∘‿2 ⥊ hasPair / i
    }
    JoinPairs ← {
      e←1+1⊏˘𝕨 ⋄ b←⊏˘𝕩  # Match end of 𝕨 (plus one) with beginning of 𝕩
      m←(≠b)>i←b⊐e      # i⊏𝕩 matches e where m is 1
      (m/𝕨) ∾˘ (m/i)⊏𝕩
    }
    # The four bracket indices for each link
    lInds ← 0‿4⊸⥊⍟(0=≠) JoinPairs○FindPairs˝ brak
    linkPos ← ⊏˘ lInds
    lInds +⎉1↩ 1‿0‿1‿0
    unused ∧↩ include ∧↩ ¬ ≠` I2M ⥊ (¯1‿1+0‿3⊸⊏)˘ lInds
    linkGroup ← 1 -˜ (1‿0⥊˜≢)⊸(/ (⊣×>)○(+`I2M) ¬⊸/) ⥊lInds
    links ← ProcLink¨˝˘ (lInds≠⊸∾2) (⊣⥊×´⊸↑) linkGroup ⊔ 𝕩

    # Emphasis (still rudimentary)
    eMasks ← (unused ∧ 𝕩⊸=)¨ "*_"
    eMasks ↩ «⊸∧¨⊸(⊣∾˜»⊸∨⊸<¨) eMasks
    eInds ← (⊢-2|⊢)∘≠⊸↑∘/¨ eMasks
    include ∧↩ ¬ I2M ∧ ∾ eInds∾1+2↓eInds
    eInds ∾↩ ⟨codeBounds⟩
    eTags ← ∾ eInds ≠⊸⥊¨ 2‿2‿1 / ("<"‿"</"∾¨·<∾⟜">")¨ "em"‿"strong"‿"code"
    eInds ↩ ∾ eInds

    # Remove backslashes used for escaping
    include ∧↩ codeMask ∨ 1 « actual

    em‿ent‿ei ← include CharEntities 𝕩
    include ∧↩ em

    add ← ∾⟨eTags,ent,code,links⟩         # Text to be added
    pos ← ∾⟨eInds,ei,codePos,linkPos⟩     # Where to add it
    ⟨include,add,pos⟩ Modify 𝕩
  }

  ######
  # Create the block structure using line classifications.

  lengths ← ≠¨ 𝕩                   # Length of each line
  blanks ← (Lead ' '⊸=)¨ 𝕩         # Number of leading blanks
  nonEmptyMask ← blanks < lengths  # Empty ←→ all leading blanks

  # Get line classifications: type of line, and data to be passed into
  # the line processor. Note that leading blanks aren't passed in.
  lineType‿lineDat ← <˘⍉ > ClassifyLine¨ blanks ↓¨ 𝕩
  # Empty lines have type ¯1.
  lineType ↩ ¯1¨⌾((¬nonEmptyMask)⊸/) lineType

  # Chase HTML comments
  commentStart ← /(lineType=5)∧lineDat=2
  EndsComment ← (3≤≠)◶0‿(∨´"-->"⍷⊢)∘(⊑⟜𝕩)
  lastCommentEnd ← ¯1
  comInd ← ∾ comments ← {
    lastCommentEnd ↩ end ← {𝕊⍟(¬EndsComment)1+𝕩}⍟(lastCommentEnd⊸<) 𝕩-1
    𝕩 + ↕end¬𝕩  # A list of indices
  }¨ commentStart
  newBlock ← (≠𝕩)↑/⁼ ⊑¨ (0<≠¨)⊸/ comments
  lineType ↩ 5¨⌾(comInd⊸⊏) lineType
  lineDat  ↩ 2¨⌾(comInd⊸⊏) lineDat

  # Lines that could be included in code blocks (will be refined)
  codeMask ← nonEmptyMask ∧ (lineType ≠ 5) ∧ blanks ≥ 4
  paragraphMask ← 0 = lineType
  # A header can't have 4 spaces of indentation. If it doesn't become
  # part of a code block, it will be included in a paragraph.
  lineType -↩ codeMask ∧ 1 = lineType

  # Tables are made up of rows that would otherwise be paragraph rows.
  # They are indicated by the delimiter row, consisting of only a few
  # allowed characters, preceded (!) by a header row with the same
  # number of cells.
  IsTD ← (∧´ ∊ ∾ ⊣ ∊˜ 2↑⊢)⟜"-|: "
  tableMask ← (0⌾⊑ nonEmptyMask) ∧ paragraphMask ∧¬ codeMask
  tableDelimMask ← { 𝕩 IsTD¨∘⊣⌾(𝕨⊸/) 𝕨 }⟜𝕩 tableMask
  delimValid ← (⊢ =○(≠∘⊔∘CutTableRow¨ ⊏⟜𝕩) -⟜1) / tableDelimMask
  headerMask ← « delimValid⌾(tableDelimMask⊸/) 0¨𝕩
  tableMask ↩ headerMask (⊢ ∧ ⊣ ∨ ⊣ PrecedesGroup <) tableMask
  lineType ↩ 3¨⌾(tableMask⊸/) lineType

  # Code blocks consist of indented lines, possibly with blank lines
  # in between. They must be separated from paragraphs by blank lines.
  codeMask ∧↩ ¬ paragraphMask PrecedesGroup codeMask
  codeMask ∨↩ codeMask (⊢ ∧ PrecedesGroup ∧ PrecedesGroup⌾⌽) lineType < 0
  lineType ↩ 2¨⌾(codeMask⊸/) lineType

  # Lines continue blocks if they are part of the same multi-line
  # type as the previous line, and otherwise start new ones.
  # Headers (type 1) always start new blocks.
  newBlock ∨↩ 1 = lineType
  blockStart ← nonEmptyMask ∧ newBlock ∨ ¯1⊸»⊸≠ lineType
  # Headers and paragraphs ignore leading blanks.
  drop ← blanks × lineType < 2
  # Group blocks based on blockStart, with type ¯1 lines excluded.
  blocks ← (1 -˜ (lineType ≥ 0) × +`blockStart) ⊔ drop ↓¨ 𝕩

  # To process a block, pick the appropriate function from procFns.
  ProcBlock ← {t‿l G b: f←t⊑procFns ⋄ l F ⊑b }
  b ← (blockStart / lineType≍˘lineDat) <∘ProcBlock˘ blocks
  JoinLines b ∾ useRepl / ""‿loadRepl
}


################################
# Testing
# Uses the test cases at https://spec.commonmark.org/0.29/spec.json
# since Github doesn't seem to have published theirs
TestSections ← {
  tests ← ¯2 ↓˘ 8⊸(÷˜⟜≠∾⊣)⊸⥊2↓•LNS •path∾"spec.json"
  tests ↩ ((⊑2+⊐⟜':')¨∘⊏ ((-','=¯1⊑⊢)↓↓)¨⎉1 ⊢) tests
  testSection ← (1↓¯1↓⊢)¨ 5⊏˘tests
  UnEscape ← {
    EscapeChar ← { ("\""tn"⊐𝕩) ⊏ "\"""∾@+9‿10 }
    esc ← IsEscaped 𝕩
    (¬«esc) / EscapeChar⌾(esc⊸/) 𝕩
  }
  RunTest ← {
    in‿exp ← UnEscape∘(1↓¯1↓⊢)¨2↑𝕩
    out ← 0 Markdown (@+10) ((⊢-˜¬×+`)∘=⊔⊢) in
    ⟨exp≡out,in,exp,out,2⊑𝕩⟩
  }

  ignore ← (2 ⊏˘ tests) ∊ ⟨"47","85"⟩
  res ← 1 ↓˘ (¬⊏˘)⊸/ RunTest˘ tests /˜ ignore < testSection ∊ 𝕩
  res
}


################################
# Syntax highlighting

# Characters in identifiers. These are also used in ProcCode to detect
# if a statement is an assignment.
idChars ← ⟨
  ('0'+↕10)∾"¯.π∞"
  "𝕣"∾˜'a'+↕26
  'A'+↕26
  "_"
⟩

# Return BQN highlights for an string 𝕩, as an ⟨add,pos⟩ list for Modify
# (include will be all 1s).
GetHighlights ← {
  # Characters used by BQN, and the HTML class they are associated with.
  classes‿chars ← <˘ ⍉ ∘‿2⥊⟨
    0             , " "∾@+9‿10  # Should never be highlighted
    "Value"       , ¯1⊏˘5‿2⥊"𝕨𝕩𝕗𝕘𝕤"# Hack around UTF-16
    "Function"    , "+-×÷⋆√⌊⌈|¬∧∨<>≠=≤≥≡≢⊣⊢⥊∾≍↑↓↕«»⌽⍉/⍋⍒⊏⊑⊐⊒∊⍷⊔!"∾¯1⊏˘5‿2⥊"𝕎𝕏𝔽𝔾𝕊"
    "Modifier"    , "˙˜˘¨⌜⁼´˝`"
    "Modifier2"   , "∘○⊸⟜⌾⊘◶⎉⚇⍟⎊"
    "Number"      , ∾idChars       # Will be classified among ↑↑ later
    "Gets"        , "←↩→"
    "Paren"       , "()"
    "Bracket"     , "⟨⟩"
    "Brace"       , "{}"
    "Ligature"    , "‿"
    "Nothing"     , "·"
    "Separator"   , "⋄,"
    "String"      , "'""@"
    "Comment"     , "#"
  ⟩
  # Turn non-whitespace classes into ⟨open,close⟩ html tags.
  classTag ← ""‿"" ∾ > {⟨"<span class='"∾𝕩∾"'>","</span>"⟩}¨ 1↓classes

  # Find each character's group, sending unknowns to 1.
  col ← (≠chars) (⊢--⟜1×=) chars FindGroup 𝕩

  # Locate comments and strings.
  c ← 𝕩='#'
  le← /(𝕩=lf)∾1
  # Line endings (le) end every comment (/c) on the line, so take a copy
  # for each # before that line but not the previous.
  ce← le /˜ -⟜» c/⊸⍋le
  # A single quote can only be used if there's another two places down.
  s ← /0‿0⊸«⊸∧𝕩='''
  d ← /𝕩='"'
  css ← ⟨ s   ⋄ ¯1↓d ⋄ /c ⟩ # Comment or string start
  cse ← ⟨ 2+s ⋄  1↓d ⋄ ce ⟩ # Corresponding end indices
  # Now b is a table of (start,end) pairs
  b ← css Trace○∾ cse
  # Given a list of pairs, get a mask indicating included regions
  ToMask ← (≠`∨⊢) (≠𝕩)↑/⁼∘∾
  # Split rows and group into text‿comments
  tc ← ((⊏˘b)⊏c) 2{𝕗↑⊔○(∾⟜𝕗)} <˘b
  # Color with "String" and "Comment"
  col ⌈↩ +´ (2‿1-˜≠classes) × ToMask¨ tc

  # UTF-16 hack: first half of a special name needs to match the second
  col↩ («col) ⊣⌾((𝕩=⊑"𝕩")⊸/) col

  # Color numeric literals and identifiers
  id ← col=5                 # ←→ 𝕩∊idChars
  w  ← »⊸< id                # Word (identifier or number) beginning mask
  wt ← idChars FindGroup w/𝕩 # Type based on first character
  wt+↩ '_' = («⊸<id)/𝕩       # Modifier1 to Modifier2 based on word end
  wt+↩ 5×0=wt                # Shift 0 to Number
  wi ← 1-˜+`id/w             # Index of word containing each of /id
  col↩(wi⊏wt)⌾(id⊸/) col

  # Tags are placed at boundaries between different colors
  boundary ← ¯1⊸»⊸≠ col
  bcol ← boundary / col
  # Windows gives us rows of start,end where the end position of one
  # color is the start of the next
  # Subtract one to place before the starting character
  pos ← 2 ↕ 1-˜/boundary∾1
  # Remove class 0 regions, as these don't use tags
  (⥊ (0<bcol)⊸/)¨ ⟨bcol⊏classTag, pos⟩
}

# Return highlights for areas in 𝕩 where 𝕨 is true.
GetMultiHighlights ← {
  start ← »⊸< 𝕨
  groups ← (1 -˜ 𝕨 × +` start) ⊔ 𝕩
  <∘∾˘ ⍉ ((≠∾2˙) ⥊ ·> (/start) {𝕨⊸+⌾(1⊸⊑)𝕩}⟜GetHighlights¨ ⊢) groups
}


################################
# Format an array to a character matrix
# Won't work on functions until we can catch errors
fmtm ← ⟨•path∾"src/fmt.bqn"⟩ •EX •path∾"dzref"
Fmt ← ⍕ _fmtm


################################
# Creating HTML files
ConvertFile ← {
  MatchStart‿MatchEnd ← { ≤○≠◶0‿(⊣ ≡ (𝕩×≠)⊸↑) }¨ 1‿¯1

  # Input file 𝕩 should be markdown; output file is fileout
  ! ".md" MatchEnd 𝕩
  fileout ← ".html" ∾˜ (¯6⊸↓∾"index"˙)⍟("README"⊸MatchEnd) ¯3↓𝕩

  # Contents of file to convert
  md ← •LNS •path∾𝕩
  # Verify and remove the html link line: the output *is* the html file.
  ! ("*View this file"⊸MatchStart ∧ (siteURL∾fileout∾").*")⊸MatchEnd) ⊑md
  out ← 𝕩 Markdown 2↓md

  parts ← (1-˜·(¬×1++`)'/'⊸=)⊸⊔ (⊑⊐⟜".")⊸↑ 𝕩
  root ← ⊑ up ← ⥊∘/⟜≍⟜"../"¨ ⌽↕≠parts
  isInd ← "README" ≡ ¯1⊑parts
  RQ ← {'"'¨⌾(('''=𝕩)⊸/)𝕩}
  Link ← RQ {∾⟨"<link href='",root,𝕩,"' rel='",𝕨,"'/>"⟩}
  head ← "head" Html lf∾JoinLines "  "⊸∾¨⟨
    "shortcut icon' type='image/x-icon" Link "favicon.ico"
    "stylesheet" Link "style.css"
    "title" Html "BQN: " ∾⍟(¬·∨´"BQN"⍷⊢) 2↓⊑(2≤≠)◶0‿("# "≡2⊸↑)¨⊸/md
  ⟩
  repo ← "a href='"‿repoURL‿"'" ∾⊸Html "BQN"
  crumbs ← up ("a href='"∾∾⟜"index.html'")⊸Html¨○((-isInd)⊸↓) (<"main")»parts
  nav ← RQ "div class='nav'" Html 3↓∾ " / "⊸∾¨ repo <⊸∾ crumbs
  front ← head ∾○(∾⟜lf) nav
  ("docs/"∾fileout) •LNS front ∾ out
}