hub:
  repo_type: generator
  builtin: l-systems
  tags:
  - banzhaf-yamamoto
chemistries:
- id: l-systems
  name: L-systems
  intuition: 'Growth rather than chemistry. A word over an alphabet is rewritten by applying productions
    to *every* letter simultaneously - which is what distinguishes it from a grammar that rewrites one
    symbol at a time, and what makes it a model of cells dividing in parallel. Interpret the resulting
    letters as turtle commands and branching plants appear; the algae rule a -> ab, b -> a generates Fibonacci
    word lengths. There is no conservation and no kinetics, which is honest: it models development, not
    reactions.

    '
  aliases:
  - Lindenmayer systems
  - D0L-systems
  - 0L-systems
  - IL-systems
  - bracketed L-systems
  - stochastic L-systems
  origin: Lindenmayer, 1968; Prusinkiewicz & Lindenmayer, 1990
  book: '9.8'
  refs:
  - '[512]'
  - '[677]'
  - '[376]'
  - '[443]'
  - http://algorithmicbotany.org/papers/abop/abop.pdf
  family: rewriting
  kind: formalism
  constructive: true
  S:
    repr: 'words over an alphabet whose symbols are one character plus an optional _subscript (a_r, F_b);
      brackets delimit branches and F f + - are turtle commands. reading words: species w<i> in discovery
      order, structure the word; reading symbols: species are the letters

      '
    definition: implicit
  R:
    definition: explicit
    arity: 1
    scheme: 'productions applied in PARALLEL to every letter of the word (unlike Chomsky grammars, which
      rewrite sequentially): a -> ab (0L), b < a -> b and F_a > F_b -> F_b (1L), 0 < 1 > 0 -> 1[+F1F1]
      (2L), F -> F[+F]F : 0.33 (stochastic). Letters without an applicable production are replaced by
      themselves. reading words: one reaction w -> w'' per derivation step (per distinct successor word
      for a stochastic system); reading symbols: one reaction a -> multiset(chi) per production

      '
    notes: 'Context matching (ABOP 1.8): symbols of #ignore are skipped; a left-context search leaves
      branches at ''['' and skips complete branches at '']''; a right-context search skips lateral branches,
      and '']'' in a right context skips the rest of the current branch. Context-sensitive productions
      take precedence over context-free ones.

      '
  A:
    reactor: maximally-parallel
    dilution: none
    notes: a derivation step rewrites the whole word at once; a word derivation is a chain of unimolecular
      reactions
  params:
  - name: system
    type: enum
    default: algae
    choices:
    - algae
    - anabaena
    - koch-snowflake
    - koch-island
    - quadratic-koch-island
    - quadratic-snowflake
    - islands-and-lakes
    - koch-variant-a
    - koch-variant-b
    - koch-variant-c
    - koch-variant-d
    - koch-variant-e
    - koch-variant-f
    - dragon-curve
    - sierpinski-gasket
    - plant-a
    - plant-b
    - plant-c
    - plant-d
    - plant-e
    - plant-f
    - stochastic-plant
    - signal-propagation
    - acropetal-signal
    - basipetal-signal
    - square-root-growth
    - hogeweg-hesper-a
    - hogeweg-hesper-b
    - hogeweg-hesper-c
    - hogeweg-hesper-d
    - hogeweg-hesper-e
    - custom
    role: structural
    meaning: 'published system from ABOP chapter 1: algae (eq. 1.2), anabaena (eq. 1.1), koch-snowflake
      (fig. 1.1 construction), koch-island (fig. 1.6), quadratic-koch-island and quadratic-snowflake (fig.
      1.7), islands-and-lakes (fig. 1.8), koch-variant-a..f (fig. 1.9), dragon-curve and sierpinski-gasket
      (fig. 1.10), plant-a..f (fig. 1.24), stochastic-plant (sec. 1.7), signal-propagation (sec. 1.8),
      acropetal-signal and basipetal-signal (fig. 1.30), square-root-growth (eq. 1.5), hogeweg-hesper-a..e
      (fig. 1.31); custom uses axiom, productions, ignore and angle'
  - name: reading
    type: enum
    default: words
    choices:
    - words
    - symbols
    role: structural
    meaning: 'words: species are the derivation words and each step is a reaction w -> w''; symbols: species
      are letters and each production a -> chi is a reaction (ABOP''s growth matrix Q; context-free systems
      only)'
  - name: axiom
    type: str
    default: ''
    range: e.g. 'F-F-F-F' or 'a_r'
    role: population
    meaning: 'custom only: the nonempty starting word'
  - name: productions
    type: list
    default: []
    range: 'e.g. [''a -> ab'', ''b -> a''], [''b < a -> b'', ''b -> a''], [''F -> F[+F]F : 0.5'', ''F
      -> F[-F]F : 0.5'']'
    role: structural
    meaning: 'custom only: productions ''l < a > r -> successor : probability'' (contexts, ''*'' and the
      probability optional; ''ε'' or nothing for the empty successor); alternatives for the same predecessor
      and context need probabilities summing to 1'
  - name: ignore
    type: str
    default: ''
    range: ABOP 1.8 uses '+-' and '+-F'
    role: structural
    meaning: 'custom only: symbols skipped during context matching (#ignore)'
  - name: angle
    type: float
    default: 0.0
    min: 0.0
    max: 360.0
    role: spatial
    meaning: 'custom only: turtle angle increment in degrees, recorded in extras.turtle (0 = none); named
      systems record their published angle'
  - name: iterations
    type: int
    default: 5
    min: 0
    max: 200
    range: 'published derivation lengths: 2-10 for the curves, 4-7 for the plants, 24-30 for Hogeweg-Hesper
      (extras.turtle.published_iterations)'
    role: population
    meaning: 'derivation length n: words up to n steps from the axiom (reading symbols: steps of extras.analysis.counts)'
  - name: max_species
    type: int
    default: 2000
    min: 1
    max: 100000
    range: 'stochastic-plant: 301 words after 2 steps; 3 steps exceed any practical budget'
    role: structural
    meaning: 'reading words: budget of words; a word with more combinations of stochastic choices than
      this is not expanded (status truncated)'
  - name: max_length
    type: int
    default: 100000
    min: 1
    max: 10000000
    role: structural
    meaning: 'reading words: successor words longer than this many symbols are not produced (status truncated)'
  provides:
  - topology
  - stoichiometry
  - catalysts
  - initial-state
  - sequence-structure-function
  fidelity: reconstructed
  sources:
  - 'Banzhaf, W. & Yamamoto, L. (2015). Artificial Chemistries, section 9.8 (one paragraph: parallel versus
    sequential rewriting, recursion, turtle interpretation; no algorithm or example).'
  - 'Prusinkiewicz, P. & Lindenmayer, A. (1990). The Algorithmic Beauty of Plants. Springer (book [677]).
    Chapter 1: D0L definition and derivation (sec. 1.2, fig. 1.3, eq. 1.1 Anabaena), turtle interpretation
    and Koch curves (sec. 1.3, figs. 1.1, 1.6-1.9), edge rewriting (fig. 1.10), bracketed 0L (sec. 1.6.3,
    fig. 1.24), stochastic 0L (sec. 1.7), context-sensitive and bracketed context matching (sec. 1.8,
    figs. 1.29-1.31), growth functions (sec. 1.9, eqs. 1.2 and 1.5, Pascal table). http://algorithmicbotany.org/papers/abop/abop.pdf'
  - 'Prusinkiewicz, P. (1986). Graphical applications of L-systems. Proceedings of Graphics Interface
    ''86, 247-253 (pL-systems: productions are tried in order, identity productions appended). http://algorithmicbotany.org/papers/graphical.gi86.pdf'
  decisions:
  - Book 9.8 only describes L-systems in prose, so the formalism and every example come from ABOP chapter
    1. Productions printed inside figures (1.7-1.10, 1.24, 1.31) are lost by pdftotext and were read from
    rendered page images; figure 1.7a's successor is printed on two lines and is F+FF-FF-F-F+F+FF-F-F+F+FF+FF-F.
  - 'Chemical reading: species are the words and each derivation step is a unimolecular reaction w ->
    w'' (reading words, default); letters as species with one reaction per production is offered as reading
    symbols. The latter''s stoichiometry is exactly ABOP''s growth matrix Q (sec. 1.9), but it loses order
    and context, so context-sensitive systems raise an error there.'
  - Species ids are w0, w1, ... in derivation (breadth-first) order with the word as structure, because
    words grow exponentially and can be empty; words are deduplicated, so a derivation that cycles closes
    into a loop. The network is built with chemart.expand.expand (alternatives=True), cut at `iterations`
    steps from the axiom; status complete means every derivation of length <= iterations is present.
  - A word that derives itself (a fixed point such as the end of the signal propagation) is an elastic
    step, dropped as in chemart.expand; its probability is kept in extras.stationary so that the alternatives
    of every expanded word still sum to 1.
  - 'Stochastic 0L (ABOP 1.7): each letter occurrence chooses its production independently, so a word
    with k rewritable letters and 3 alternatives each has 3^k successor combinations. All distinct successor
    words become separate reactions; a word''s probability is the sum over the combinations giving it.
    Probabilities are not rates (no kinetics is defined), so they are stored in extras.probabilities aligned
    with the reactions and the reactions carry rate None. max_species also caps the number of combinations
    enumerated for one word (status truncated), since stochastic-plant reaches 3^25 combinations at step
    3.'
  - 'Nondeterministic 0L-systems without probabilities are not accepted: alternatives for the same predecessor
    and context must carry probabilities summing to 1 (ABOP''s pi).'
  - 'Production notation is Chemart''s text form of ABOP''s: ''l < a > r -> chi : p'' with spaces around
    < and >, ''*'' for no context (as in fig. 1.31) and the probability after '' : '' (ABOP prints it
    over the arrow). Subscripted letters are written with an underscore (a_r, F_b). The strict predecessor
    is one symbol; brackets replace themselves and cannot be rewritten; the axiom and successors must
    have balanced brackets.'
  - 'Context matching follows ABOP 1.8 and figure 1.29 (BC < S > G[H]M matches S in ABC[DE][SG[HI[JK]L]MNO]):
    left search skips ignored symbols, ''['' and complete bracketed branches; right search skips ignored
    symbols and lateral branches, and '']'' in the right context skips the rest of the branch. Among several
    matching context-sensitive productions for one letter, the first listed applies (GI''86 pL-system
    ordering); ABOP does not say.'
  - 'koch-snowflake (axiom F--F--F, F -> F+F--F+F, 60 degrees) is not printed as an L-system in ABOP chapter
    1: it is the coding of the figure 1.1 construction (generator of 4 sides of length 1/3) that section
    1.3 describes (initiator = axiom, generator = successor). Every other named system is transcribed
    from ABOP.'
  - 'algae: ABOP eq. (1.2) is a -> ab, b -> a from axiom a (the popular ''Lindenmayer algae'' A -> AB,
    B -> A); figure 1.3 runs the same productions from axiom b. Lindenmayer (1968) [512] (J. Theor. Biol.,
    paywalled) was not consulted; the examples and their numbers are verified against ABOP only.'
  - The eq. (1.5) growth function prints as 'fG(n) = sqrt n + 4' with the floor brackets lost; the derivation
    gives floor(sqrt(n)) + 4 symbols (X and the F's), which is what the test checks.
  - 'Turtle interpretation is not executed: the published angle and derivation length of each named system
    are recorded in extras.turtle (angle, published_iterations); ABOP gives no step length d. The v1 ''space''
    capability is removed because no positions are generated, and v1 ''stochastic'' is dropped (probabilities
    in the productions make a system stochastic).'
  - Parametric L-systems (ABOP 1.10), 3D turtle symbols and the growth of Anabaena with heterocysts (L-system
    1.1) are not implemented; [376] (recurrence systems) and [443] (virtual cities) are cited by the book
    as context only.
  phenomena:
  - 'parallel rewriting: the algae system a -> ab, b -> a gives a, ab, aba, abaab, abaababa with Fibonacci
    word lengths; from axiom b it gives b, a, ab, aba, abaab, abaababa, abaababaabaab (ABOP fig. 1.3)'
  - 'Anabaena catenula filament (ABOP eq. 1.1): a_r, a_l b_r, b_l a_r a_r, a_l a_l b_r a_l b_r, b_l a_r
    b_l a_r a_r b_l a_r a_r, cell counts following the Fibonacci numbers'
  - 'Koch constructions: the Koch curve has 4^n segments after n steps, the quadratic Koch island 4 *
    8^n (fig. 1.6), the quadratic snowflake 5^n (fig. 1.7b)'
  - 'growth functions (ABOP 1.9): exponential for F -> FF (2^n), Fibonacci for eq. 1.2, polynomial with
    the Pascal triangle for a_i -> a_i a_{i+1}, floor(sqrt(n)) + 4 for the 2L-system of eq. 1.5'
  - 'context-sensitive signal propagation (ABOP 1.8): baaaaaaaa, abaaaaaaa, aabaaaaaa, ...; acropetal
    signals reach every segment above, basipetal signals only the path to the root'
  - stochastic L-systems give different specimens of the same plant species; the successor words of any
    word have probabilities summing to 1
  notes: 'Growth/development rather than chemistry: no conservation and no kinetics, and a derivation
    step is unimolecular. The module also exports LSystem (successors, derive, derivation), parse_production,
    symbols and SYSTEMS for direct use.

    '