dsp_toolkit example¶

A two-component DSP library built with just-makeit: a Gain component and an Ema (exponential moving average) component.

Follow along to scaffold, implement, and combine them — and see the one place the generator currently needs a manual touch when you add a second component.

TL;DR — see it work first¶

. <(curl -fsSL https://just-buildit.github.io/just-makeit/install.sh)
just-makeit example dsp_toolkit
# dsp_toolkit: PASSED

Prerequisites¶

. <(curl -fsSL https://just-buildit.github.io/just-makeit/install.sh)

Pass a custom path to keep the venv somewhere persistent:

. <(curl -fsSL https://just-buildit.github.io/just-makeit/install.sh) -- ~/my-venv

Or with pip if just-makeit is already installed:

pip install just-makeit && just-makeit install-deps
source /tmp/jm-venv/bin/activate

1. Scaffold¶

just-makeit new dsp_toolkit \
    --object gain \
    --arg-type float \
    --return-type float \
    --state "gain:float:1.0"
cd dsp_toolkit && make

Scaffolds a single gain component with a real-valued step():

Name	Type	Default	Role
`gain`	`float`	`1.0`	Scalar gain

make configures CMake and builds the extension. The C test already passes before you write a single line of logic.

2. Implement gain¶

Open native/inc/gain/gain_core.h and replace the gain_step stub — it's a one-liner:

static inline float
gain_step (const gain_state_t *state, float x)
{
  return state->gain * x;
}

gain_steps() in gain_core.c loops over this automatically — no changes needed there.

3. Add a second component¶

just-makeit object ema \
    --arg-type float \
    --return-type float \
    --state "alpha:double:0.1" \
    --state "prev:float:0.0"

just-makeit object adds ema alongside gain in the same project:

new C header, source, test, and benchmark under native/
new Python stub, test, and benchmark under src/dsp_toolkit/
umbrella header native/inc/dsp_toolkit.h updated with #include "ema/ema_core.h"
root CMakeLists.txt updated with add_subdirectory and target_link_libraries

State:

Name	Type	Default	Role
`alpha`	`double`	`0.1`	Smoothing factor (0 \< α \< 1)
`prev`	`float`	`0.0`	Previous output sample

4. Implement ema¶

ema_step must write back to state->prev, so the signature drops const:

static inline float
ema_step (ema_state_t *state, float x)
{
  float y
      = (float)state->alpha * x + (float)(1.0 - state->alpha) * state->prev;
  state->prev = y;
  return y;
}

The patch script handles both the body replacement and the const removal:

"""Patch ema_step stub with the implementation.

EMA writes back to state->prev, so the signature drops `const`:
  const ema_state_t *  ->  ema_state_t *

Run from the project root: python3 .steps/04_patch.py
"""

import pathlib
import re
import sys

header = pathlib.Path("native/inc/ema/ema_core.h")
impl = pathlib.Path(__file__).with_name("04_step_after.c")

stub_re = re.compile(
    r"(static inline|JM_FORCEINLINE JM_HOT) float\s*\n"
    r"ema_step\((?:const )?ema_state_t \*state.*?\n\}",
    re.DOTALL,
)

text = header.read_text()
m = stub_re.search(text)
if not m:
    print(
        "ERROR: stub not found — already patched or file changed",
        file=sys.stderr,
    )
    sys.exit(1)

qualifier = m.group(1)
# The impl already has the non-const signature; sub replaces the whole match.
replacement = impl.read_text().strip().replace("static inline", qualifier, 1)
patched = stub_re.sub(replacement, text)
header.write_text(patched)
print(f"patched {header}")

5. Both components are exported automatically¶

After just-makeit object, __init__.py is updated in-place — the new import and __all__ entry are spliced in without touching anything else:

"""dsp_toolkit — Gain component."""

from .gain import Gain
from .ema import Ema

__all__ = ["Gain", "Ema"]

Existing imports and any user additions are preserved.

6. Build and test¶

make && make test

CTest runs test_gain_core and test_ema_core. pytest runs the 14 generated tests across both components.

7. Use from Python¶

"""Demo: use Gain and Ema together from Python."""

import sys

sys.path.insert(0, "src")

import numpy as np
from dsp_toolkit import Gain, Ema

# A short burst followed by silence
signal = np.ones(20, dtype=np.float32)
signal[10:] = 0.0

gain = Gain(gain=2.0)
ema = Ema(alpha=0.3)

print(f"{'n':>3}  {'input':>7}  {'gained':>7}  {'smoothed':>10}")
print("-" * 36)
for i, x in enumerate(signal):
    y_gain = gain.step(x)
    y_ema = ema.step(float(y_gain))
    print(f"{i:>3}  {x:>7.3f}  {y_gain:>7.3f}  {y_ema:>10.4f}")

Gain and Ema are independent stateful objects — chain them however you need. steps() is also available for block processing:

y = Gain(gain=2.0).steps(signal)   # returns float32 ndarray

8. Use from Python¶

"""Demo: use Gain and Ema together from Python."""

import sys

sys.path.insert(0, "src")

import numpy as np
from dsp_toolkit import Gain, Ema

# A short burst followed by silence
signal = np.ones(20, dtype=np.float32)
signal[10:] = 0.0

gain = Gain(gain=2.0)
ema = Ema(alpha=0.3)

print(f"{'n':>3}  {'input':>7}  {'gained':>7}  {'smoothed':>10}")
print("-" * 36)
for i, x in enumerate(signal):
    y_gain = gain.step(x)
    y_ema = ema.step(float(y_gain))
    print(f"{i:>3}  {x:>7.3f}  {y_gain:>7.3f}  {y_ema:>10.4f}")

Gain and Ema are independent stateful objects — chain them however you need. steps() is also available for block processing:

y = Gain(gain=2.0).steps(signal)   # returns float32 ndarray