This project is part of the @thi.ng/umbrella monorepo.

This package provides a single function, a general purpose Finite State Machine transducer, which acts as useful & lightweight mechanism to provide context-sensitive processing capabilities as part of a transducer transformation pipeline.

yarn add @thi.ng/transducers-fsm

• @thi.ng/api
• @thi.ng/transducers

For a real world example, the @thi.ng/sax package provides a SAX-like XML parser transducer, built around the FSM provided here.

The following example defines a simple FSM with 3 states:

• skip
• take
• done

The FSM always starts in the skip state.

The FSM alternates between skipping or consuming (passing through) 5 inputs as long as each input is < 20. Once an input is >= 20, the FSM switches into the done state, which has been declared as a terminal state and once entered will cause processing to terminate (also see API description further below).

import { fsm } from '@thi.ng/transducers-fsm'
import * as tx from '@thi.ng/transducers'
import { isOdd } from '@thi.ng/checks'

const testFSM = fsm({

    // initial state initializer
    // (called before processing 1st input)
    init: () => ({ state: "skip", count: 0 }),

    // terminal state ID
    terminate: "done",

    // individual state handlers
    states: {
        // skip state
        skip: (state, x) => {
            if (x < 20) {
                if (++state.count > 5) {
                    state.state = "take";
                    state.count = 1;
                    return [x];
                }
            } else {
                state.state = "done";
            }
        },

        // take state
        take: (state, x) => {
            if (x < 20) {
                if (++state.count > 5) {
                    state.state = "skip";
                    state.count = 1;
                } else {
                    return [x];
                }
            } else {
                state.state = "done";
            }
        },

        // terminal state, ignore inputs
        done: () => { },
    },
});

[...tx.iterator(testFSM, tx.range(100))]
// [ 5, 6, 7, 8, 9, 15, 16, 17, 18, 19 ]

// Use FSM as part of composed transducers...

[...tx.iterator(tx.comp(tx.takeNth(2), testFSM), tx.range(100))]
// [ 10, 12, 14, 16, 18 ]

[
    ...tx.iterator(
        tx.comp(
            tx.mapcat((x) => x.split(/[,\s]+/g)),
            tx.map((x) => parseInt(x)),
            testFSM,
            tx.filter(isOdd)
        ),
        ["9,8,7,6", "14 1 0 17 15 16", "19,23,12,42,4"]
    )
]
// [ 1, 17, 15 ]

Finite State Machine transducer. Takes an FSM configuration object and returns a transducer, which processes inputs using the provided state handler functions, which in turn can produce any number of outputs per consumed input.

Before processing the first input, the FSM state is initialized by calling the user provided init() function, which MUST return a state object with at least a state key, whose value is used for dynamic (i.e. stateful) dispatch during input processing. This state object is passed with each input value to the current state handler, which is expected to mutate this object, e.g. to cause state changes based on given inputs.

If a state handler needs to "emit" results for downstream processing, it can return an array of values. Any such values are passed on (individually, not as array) to the next reducer in the chain. If a state handler returns null or undefined, further downstream processing of the current input is skipped.

Regardless of return value, if a state handler has caused a state change to the configured terminate state, processing is terminated (by calling ensureReduced()) and no further inputs will be consumed.

• Karsten Schmidt

© 2018 Karsten Schmidt // Apache Software License 2.0