Amoeba -- the constraint solving algorithm in pure C

Amoeba is a pure C implement of Cassowary algorithm. Amoeba use Clean C, which is the cross set of ANSI C89 and C++, like the Lua language.

Amoeba is a single-file library, for more single-file library, see the stb project here.

Amoeba largely impressed by kiwi, the C++ implement of Cassowary algorithm, and the algorithm paper.

Amoeba ships a hand written Lua binding.

Amoeba has the same license with the Lua language.

How To Use

This libary export a constraint solver interface, to solve a constraint problem, you should use it in steps:

• create a new solver: am_newsolver()
• create some variables: am_newvariable()
• create some constraints that may use variables: am_newconstraint()
• make constraints by construct equation using:
  • am_addterm() add a $a \times variable$ term to constraint equation items
  • am_setrelation() to specify the equal/greater/less sign in center of equation
  • am_addconstant() to add a number without variable
  • am_setstrength() to specify the priority of this constraint in all constraints
• after make up a constraint, you could add it into solver by am_add()
• and you can read out the result of each variable with am_value()
• or you can manually specify a new value to variable with am_sugguest()
• after done, use am_delsolver() to free al memory

below is a tiny example to demonstrate the steps:

#define AM_IMPLEMENTATION // include implementations of library
#include "amoeba.h"       // and interface

int main(void)
{
    // first, create a solver:
    am_Solver *S = am_newsolver(NULL, NULL);

    // create some variable:
    am_Var *l = am_newvariable(S);
    am_Var *m = am_newvariable(S);
    am_Var *r = am_newvariable(S);

    // create the constraint: 
    am_Constraint *c1 = am_newconstraint(S, AM_REQUIRED);
    am_Constraint *c2 = am_newconstraint(S, AM_REQUIRED);

    // c1: m is in middle of l and r:
    //     i.e. m = (l + r) / 2, or 2*m = l + r
    am_addterm(c1, m, 2.f);
    am_setrelation(c1, AM_EQUAL);
    am_addterm(c1, l, 1.f);
    am_addterm(c1, r, 1.f);
    // apply c1
    am_add(c1);

    // c2: r - l >= 100
    am_addterm(c2, r, 1.f);
    am_addterm(c2, l, -1.f);
    am_setrelation(c2, AM_GREATEQUAL);
    am_addconstant(c2, 100.f);
    // apply c2
    am_add(c2);

    // now we set variable l to 20
    am_suggest(l, 20.f);

    // and see the value of m and r:
    am_updatevars(S);

    // r should by 20 + 100 == 120:
    assert(am_value(r) == 120.f);

    // and m should in middle of l and r:
    assert(am_value(m) == 70.f);
    
    // done with solver
    am_delsolver(S);
    return 0;
}

Reference

All functions below that returns int may return error codes:

• AM_OK: the operations success.
• AM_FAILED: the operation fail
• AM_UNSATISFIED: can not add specific constraints into solver
• AM_UNBOUND: add specific constraints failed because variables in constraints unbound

Routines:

• am_Solver *am_newsolver(am_Allocf *allocf, void *ud);

  create a new solver with custom memory alloculator, pass NULL for use default ones.

• void am_resetsolver(am_Solver *solver, int clear_constraints);

  remove all variable suggests from solver.

  if clear_constraints is nonzero, also remove and delete all constraints from solver.

• void am_delsolver(am_Solver *solver);

  delete a solver and frees all memory it used, after that, all variables/constraints created from this solver are all freed.

• void am_updatevars(am_Solver *solver);

  refresh variables' value into it's constrainted value, you could use am_autoupdate() to avoid call this routine every time on changing constraints in solver.

• void am_autoupdate(am_Solver *solver, int auto_update);

  set auto update flags, if set, all variable will auto update to its' latest value after any changes to solver.

• int am_hasedit(am_Var *var);

  check whether a variable has suggested value in solver.

• int am_hasconstraint(am_Constraint *cons);

  check whether a constraint has been added into solver.

• int am_add(am_Constraint *cons);

  add constraint into solver it's created from.

• void am_remove(am_Constraint *cons);

  remove added constraint.

• int am_addedit(am_Var *var, am_Num strength);

  prepare to change the value of variables or the strength value if the variable is in edit now.

• void am_suggest(am_Var *var, am_Num value);

  actually change the value of the variable var, after changed, other variable may changed due to the constraints in solver. if you do not want change the strength of suggest (default is AM_MEDIUM), you may call this routine directly.

• void am_deledit(am_Var *var);

  cancel the modify of variable, the value will restore to the referred value according the solver.

• am_Var *am_newvariable(am_Solver *solver);

  create a new variable. variable is reference counting since it may used in serveral constraints,

  so if you want store it in multiple place, call am_usevariable() before.

• void am_usevariable(am_Var *var);

  add the reference counting of a variable to avoid it been freed.

• void am_delvariable(am_Var *var);

  sub the reference counting of a variable, and free it when the count down to 0.

• int am_variableid(am_Var *var);

  return a unqiue id (within solver) of the variable var.

• am_Num am_value(am_Var *var);

  fetch the current value of variable var, note that if auto update not set and am_updatevars() not called, the value may not the latest values that inferred by solver.

• am_Constraint *am_newconstraint(am_Solver *solver, am_Num strength);

  create a new constraints.

• am_Constraint *am_cloneconstraint(am_Constraint *other, am_Num strength);

  make a new constraints from existing one, with new strength

• void am_resetconstraint(am_Constraint *cons);

  remove all terms and variables from the constraint.

• void am_delconstraint(am_Constraint *cons);

  frees the constraint. if it's added into solver, remove it first.

• int am_addterm(am_Constraint *cons, am_Var *var, am_Num multiplier);

  add a term into constraint, e.g. a constraint like $2m = x + y$, the terms are $2m$, $1x$ and $1y$.

  so makeup this constraint you could:

  am_addterm(c, m, 2.0); // 2*m
  am_setrelation(c, AM_EQUAL); // =
  am_addterm(c, x, 1.0); // x
  am_addterm(c, y, 1.0); // y
  

• int am_setrelation(am_Constraint *cons, int relation);

  set the relations of constraint, could be one of these:

  • AM_EQUAL
  • AM_GREATEQUAL
  • AM_LESSEQUAL

  the terms added before am_setrelation() become the left hand terms of constraints, and the terms adds after call will become the right hand terms of constraints.

• int am_addconstant(am_Constraint *cons, am_Num constant);

  add a constant without variable into constraint as a term.

• int am_setstrength(am_Constraint *cons, am_Num strength);

  set the strength of a constraint.

• am_mergeconstraint(am_Constraint *cons, const am_Constraint *other, am_Num multiplier);

  merge other constraints into cons, with a multiplier multiples with other.