Internal details

Pyoframe's inner workings involve a few tricks that one should be aware of if they wish to modify Pyoframe's internal code.

The zero variable

Whenever a Model is instantiated, Pyoframe immediately creates a variable whose value is fixed to 1 and has a variable id of 0 — the Zero Variable. This allows Pyoframe to represent constant terms in mathematical expressions as multiples of the Zero Variable. For example, the expression 3 * var_8 + 5 is represented as 3 * var_8 + 5 * var_0. This eliminates the need to separately track constant terms and also simplifies the handling of quadratics.

Quadratics

Internally, Expression is used to represent both linear and quadratic mathematical expressions. When a quadratic expression is formed, column __quadratic_variable_id is added to Expression.data. If an expression's quadratic terms happen to cancel out (e.g. (ab + c) - ab), this column is automatically removed.

Column __quadratic_variable_id records the ID of the second variable in a quadratic term (the b in 3ab). For linear terms, which have no second variable, this column contains the Zero Variable. Quadratic terms are always stored such that the first term's variable ID (in column __variable_id) is greater or equal to the second term's variable id (in column __quadratic_variable_id). For example, var_7 * var_8 would be rearranged and stored as var_8 * var_7. This helps simplify expressions and provides a useful guarantee: If the variable in the first column (__variable_id) is the Zero Variable (var_0) we know the variable in the second column must also be the Zero Variable and, thus, the term must be a constant.