expexp/src/variableScope.js

The express model io and express model and data representation.

// Provides a scope with variable names to lookup values and query whether
// a given value is by copy or by reference.
// Variable scope is hierarical, meaning that there can be a parent scope given
// on construction. That if any lookup on the current scope fails, the parent
// scope is tried for instead.
// There are two different notions about setting a value: First a value and its
// type model must be registered. This can only happen once and only for the
// current scope - not for the parent. On the other hand a value can be set
// several times and if declared as 'by reference' (proc VAR) setting can also
// affect the value in the parents scope. It is not up to this class to check
// if a value is valid to be set for a given name or type model.
export function VariableScope(parent) {
	const me = {}

	const c = {} // name cache
	const BY_VAL = -1
	const r = {} // reference index (or -1 if not a reference - mostly the case)

	const setup = function() {
		// nothing to do
	}

	// Register a value.
	// If a value is by reference (onyl case: procedure var param) value is always
	// an array and isRefIdx says what position the actual value has within that array.
	// Like that an entites attrValues might be passed (infrastructure array) as
	// value or a data arrays position.
	me.regVal = function(name, value, isRefIdx = -1) {
		c[name] = value
		r[name] = isRefIdx
	}

	me.has = function(name) {
		if (name in c) {
			return true
		} else if (parent) {
			return parent.has(name)
		} else {
			return false
		}
	}

	me.get = function(name) {
		if (name in c) {
			if (r[name] == BY_VAL) {
				return c[name]
			} else { // by reference (var)
				return getByRef(name)
			}
		} else if (parent) {
			return parent.get(name)
		}
	}

	// Set an existing value. It must already be registered.
	me.set = function(name, value) {
		if (name in c) {
			if (r[name] == BY_VAL) {
				c[name] = value
			} else { // by reference (var)
				setByRef(name, value)
			}
			return true
		} else if (parent) {
			return parent.set(name, value)
		}
	}

	const getByRef = function(name) {
		return c[name][r[name]]
	}

	const setByRef = function(name, value) {
		c[name][r[name]] = value
	}

	setup()
	return me
}