Add a simple filter example with FFTW

Cargo.lock

@@ -18,6 +18,12 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a13739d7177fbd22bb0ed28badfff9f372f8bef46c863db4e1c6248f6b223b6e"
 
+[[package]]
+name = "adler"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f26201604c87b1e01bd3d98f8d5d9a8fcbb815e8cedb41ffccbeb4bf593a35fe"
+
 [[package]]
 name = "ahash"
 version = "0.7.6"
@@ -29,6 +35,15 @@ dependencies = [
  "version_check",
 ]
 
+[[package]]
+name = "aho-corasick"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca972c2ea5f742bfce5687b9aef75506a764f61d37f8f649047846a9686ddb66"
+dependencies = [
+ "memchr 0.1.11",
+]
+
 [[package]]
 name = "anyhow"
 version = "1.0.55"
@@ -71,7 +86,7 @@ dependencies = [
  "objc",
  "raw-window-handle",
  "uuid",
- "winapi",
+ "winapi 0.3.9",
  "x11",
  "xcb 0.9.0",
  "xcb-util",
@@ -89,6 +104,33 @@ version = "0.1.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0d8c1fef690941d3e7788d328517591fecc684c084084702d6ff1641e993699a"
 
+[[package]]
+name = "byteorder"
+version = "1.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "14c189c53d098945499cdfa7ecc63567cf3886b3332b312a5b4585d8d3a6a610"
+
+[[package]]
+name = "bzip2"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6afcd980b5f3a45017c57e57a2fcccbb351cc43a356ce117ef760ef8052b89b0"
+dependencies = [
+ "bzip2-sys",
+ "libc",
+]
+
+[[package]]
+name = "bzip2-sys"
+version = "0.1.11+1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "736a955f3fa7875102d57c82b8cac37ec45224a07fd32d58f9f7a186b6cd4cdc"
+dependencies = [
+ "cc",
+ "libc",
+ "pkg-config",
+]
+
 [[package]]
 name = "cargo-nih-plug"
 version = "0.1.0"
@@ -108,6 +150,16 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
+[[package]]
+name = "chrono"
+version = "0.2.25"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9213f7cd7c27e95c2b57c49f0e69b1ea65b27138da84a170133fd21b07659c00"
+dependencies = [
+ "num",
+ "time",
+]
+
 [[package]]
 name = "clap-sys"
 version = "0.1.0"
@@ -120,7 +172,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9fdf5e01086b6be750428ba4a40619f847eb2e95756eee84b18e06e5f0b50342"
 dependencies = [
  "lazy-bytes-cast",
- "winapi",
+ "winapi 0.3.9",
 ]
 
 [[package]]
@@ -209,6 +261,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "crc32fast"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b540bd8bc810d3885c6ea91e2018302f68baba2129ab3e88f32389ee9370880d"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "crossbeam"
 version = "0.8.1"
@@ -252,7 +313,7 @@ checksum = "c00d6d2ea26e8b151d99093005cb442fb9a37aeaca582a03ec70946f49ab5ed9"
 dependencies = [
  "cfg-if",
  "crossbeam-utils",
- "lazy_static",
+ "lazy_static 1.4.0",
  "memoffset",
  "scopeguard",
 ]
@@ -274,7 +335,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b5e5bed1f1c269533fa816a0a5492b3545209a205ca1a54842be180eb63a16a6"
 dependencies = [
  "cfg-if",
- "lazy_static",
+ "lazy_static 1.4.0",
 ]
 
 [[package]]
@@ -349,6 +410,57 @@ dependencies = [
  "nohash-hasher",
 ]
 
+[[package]]
+name = "fftw"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9dc6a84bbb6bac64521bfb5a544701ce118f329d550cf9b9054d1954fe61adef"
+dependencies = [
+ "bitflags",
+ "fftw-sys",
+ "lazy_static 1.4.0",
+ "ndarray",
+ "num-complex",
+ "num-traits",
+ "thiserror",
+]
+
+[[package]]
+name = "fftw-src"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c08962470ab0e91e74ec7d338c8731476c28ed4e503a3080b0f001692e395a7c"
+dependencies = [
+ "anyhow",
+ "cc",
+ "fs_extra",
+ "ftp",
+ "zip",
+]
+
+[[package]]
+name = "fftw-sys"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e8e3951d695cc2f17610cd041e87ebc15078d1af5eb8c6be77921381fc98b3fd"
+dependencies = [
+ "fftw-src",
+ "libc",
+ "num-complex",
+]
+
+[[package]]
+name = "flate2"
+version = "1.0.22"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e6988e897c1c9c485f43b47a529cef42fde0547f9d8d41a7062518f1d8fc53f"
+dependencies = [
+ "cfg-if",
+ "crc32fast",
+ "libc",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "foreign-types"
 version = "0.3.2"
@@ -364,6 +476,23 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "00b0228411908ca8685dba7fc2cdd70ec9990a6e753e89b6ac91a84c40fbaf4b"
 
+[[package]]
+name = "fs_extra"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2022715d62ab30faffd124d40b76f4134a550a87792276512b18d63272333394"
+
+[[package]]
+name = "ftp"
+version = "3.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "542951aad0071952c27409e3bd7cb62d1a3ad419c4e7314106bf994e0083ad5d"
+dependencies = [
+ "chrono",
+ "lazy_static 0.1.16",
+ "regex",
+]
+
 [[package]]
 name = "gain"
 version = "0.1.0"
@@ -429,6 +558,16 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1aab8fc367588b89dcee83ab0fd66b72b50b72fa1904d7095045ace2b0c81c35"
 
+[[package]]
+name = "kernel32-sys"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7507624b29483431c0ba2d82aece8ca6cdba9382bff4ddd0f7490560c056098d"
+dependencies = [
+ "winapi 0.2.8",
+ "winapi-build",
+]
+
 [[package]]
 name = "keyboard-types"
 version = "0.6.2"
@@ -450,6 +589,12 @@ version = "5.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "10257499f089cd156ad82d0a9cd57d9501fa2c989068992a97eb3c27836f206b"
 
+[[package]]
+name = "lazy_static"
+version = "0.1.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf186d1a8aa5f5bee5fd662bc9c1b949e0259e1bcc379d1f006847b0080c7417"
+
 [[package]]
 name = "lazy_static"
 version = "1.4.0"
@@ -469,7 +614,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "efbc0f03f9a775e9f6aed295c6a1ba2253c5757a9e03d55c6caa46a681abcddd"
 dependencies = [
  "cfg-if",
- "winapi",
+ "winapi 0.3.9",
 ]
 
 [[package]]
@@ -499,6 +644,24 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "matrixmultiply"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "916806ba0031cd542105d916a97c8572e1fa6dd79c9c51e7eb43a09ec2dd84c1"
+dependencies = [
+ "rawpointer",
+]
+
+[[package]]
+name = "memchr"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d8b629fb514376c675b98c1421e80b151d3817ac42d7c667717d282761418d20"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "memchr"
 version = "2.4.1"
@@ -529,6 +692,29 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "68354c5c6bd36d73ff3feceb05efa59b6acb7626617f4962be322a825e61f79a"
 
+[[package]]
+name = "miniz_oxide"
+version = "0.4.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a92518e98c078586bc6c934028adcca4c92a53d6a958196de835170a01d84e4b"
+dependencies = [
+ "adler",
+ "autocfg",
+]
+
+[[package]]
+name = "ndarray"
+version = "0.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c0d5c9540a691d153064dc47a4db2504587a75eae07bf1d73f7a596ebc73c04"
+dependencies = [
+ "matrixmultiply",
+ "num-complex",
+ "num-integer",
+ "num-traits",
+ "rawpointer",
+]
+
 [[package]]
 name = "nih_plug"
 version = "0.0.0"
@@ -539,7 +725,7 @@ dependencies = [
  "cfg-if",
  "clap-sys",
  "crossbeam",
- "lazy_static",
+ "lazy_static 1.4.0",
  "nih_plug_derive",
  "parking_lot",
  "raw-window-handle",
@@ -566,7 +752,7 @@ dependencies = [
  "crossbeam",
  "egui",
  "egui-baseview",
- "lazy_static",
+ "lazy_static 1.4.0",
  "nih_plug",
  "parking_lot",
 ]
@@ -607,11 +793,61 @@ version = "7.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1b1d11e1ef389c76fe5b81bcaf2ea32cf88b62bc494e19f493d0b30e7a930109"
 dependencies = [
- "memchr",
+ "memchr 2.4.1",
  "minimal-lexical",
  "version_check",
 ]
 
+[[package]]
+name = "num"
+version = "0.1.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4703ad64153382334aa8db57c637364c322d3372e097840c72000dabdcf6156e"
+dependencies = [
+ "num-integer",
+ "num-iter",
+ "num-traits",
+]
+
+[[package]]
+name = "num-complex"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "747d632c0c558b87dbabbe6a82f3b4ae03720d0646ac5b7b4dae89394be5f2c5"
+dependencies = [
+ "num-traits",
+]
+
+[[package]]
+name = "num-integer"
+version = "0.1.44"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d2cc698a63b549a70bc047073d2949cce27cd1c7b0a4a862d08a8031bc2801db"
+dependencies = [
+ "autocfg",
+ "num-traits",
+]
+
+[[package]]
+name = "num-iter"
+version = "0.1.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b2021c8337a54d21aca0d59a92577a029af9431cb59b909b03252b9c164fad59"
+dependencies = [
+ "autocfg",
+ "num-integer",
+ "num-traits",
+]
+
+[[package]]
+name = "num-traits"
+version = "0.2.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a64b1ec5cda2586e284722486d802acf1f7dbdc623e2bfc57e65ca1cd099290"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "objc"
 version = "0.2.7"
@@ -706,7 +942,7 @@ version = "0.22.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8533f14c8382aaad0d592c812ac3b826162128b65662331e1127b45c3d18536b"
 dependencies = [
- "memchr",
+ "memchr 2.4.1",
 ]
 
 [[package]]
@@ -727,6 +963,12 @@ dependencies = [
  "cty",
 ]
 
+[[package]]
+name = "rawpointer"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60a357793950651c4ed0f3f52338f53b2f809f32d83a07f72909fa13e4c6c1e3"
+
 [[package]]
 name = "redox_syscall"
 version = "0.2.11"
@@ -742,9 +984,28 @@ version = "0.1.3"
 source = "git+https://github.com/nicokoch/reflink?rev=e8d93b465f5d9ad340cd052b64bbc77b8ee107e2#e8d93b465f5d9ad340cd052b64bbc77b8ee107e2"
 dependencies = [
  "libc",
- "winapi",
+ "winapi 0.3.9",
 ]
 
+[[package]]
+name = "regex"
+version = "0.1.80"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fd4ace6a8cf7860714a2c2280d6c1f7e6a413486c13298bbc86fd3da019402f"
+dependencies = [
+ "aho-corasick",
+ "memchr 0.1.11",
+ "regex-syntax",
+ "thread_local",
+ "utf8-ranges",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9ec002c35e86791825ed294b50008eea9ddfc8def4420124fbc6b08db834957"
+
 [[package]]
 name = "ryu"
 version = "1.0.9"
@@ -835,7 +1096,7 @@ checksum = "1325f292209cee78d5035530932422a30aa4c8fda1a16593ac083c1de211e68a"
 dependencies = [
  "bitflags",
  "dlib",
- "lazy_static",
+ "lazy_static 1.4.0",
  "log",
  "memmap2",
  "nix",
@@ -859,6 +1120,7 @@ dependencies = [
 name = "stft"
 version = "0.1.0"
 dependencies = [
+ "fftw",
  "nih_plug",
 ]
 
@@ -873,6 +1135,55 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "thiserror"
+version = "1.0.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "854babe52e4df1653706b98fcfc05843010039b406875930a70e4d9644e5c417"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aa32fd3f627f367fe16f893e2597ae3c05020f8bba2666a4e6ea73d377e5714b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "thread-id"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a9539db560102d1cef46b8b78ce737ff0bb64e7e18d35b2a5688f7d097d0ff03"
+dependencies = [
+ "kernel32-sys",
+ "libc",
+]
+
+[[package]]
+name = "thread_local"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8576dbbfcaef9641452d5cf0df9b0e7eeab7694956dd33bb61515fb8f18cfdd5"
+dependencies = [
+ "thread-id",
+]
+
+[[package]]
+name = "time"
+version = "0.1.43"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca8a50ef2360fbd1eeb0ecd46795a87a19024eb4b53c5dc916ca1fd95fe62438"
+dependencies = [
+ "libc",
+ "winapi 0.3.9",
+]
+
 [[package]]
 name = "toml"
 version = "0.5.8"
@@ -894,6 +1205,12 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ccb82d61f80a663efe1f787a51b16b5a51e3314d6ac365b08639f52387b33f3"
 
+[[package]]
+name = "utf8-ranges"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1ca13c08c41c9c3e04224ed9ff80461d97e121589ff27c753a16cb10830ae0f"
+
 [[package]]
 name = "uuid"
 version = "0.8.2"
@@ -1021,7 +1338,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d9341df79a8975679188e37dab3889bfa57c44ac2cb6da166f519a81cbe452d4"
 dependencies = [
  "dlib",
- "lazy_static",
+ "lazy_static 1.4.0",
  "pkg-config",
 ]
 
@@ -1031,6 +1348,12 @@ version = "1.0.0-beta.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e6f1efe828a707edf85994a4501734ac1c1b9d244cfcf4de235f11c4125ace8f"
 
+[[package]]
+name = "winapi"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"
@@ -1041,6 +1364,12 @@ dependencies = [
  "winapi-x86_64-pc-windows-gnu",
 ]
 
+[[package]]
+name = "winapi-build"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"
+
 [[package]]
 name = "winapi-i686-pc-windows-gnu"
 version = "0.4.0"
@@ -1181,3 +1510,17 @@ version = "0.1.0"
 dependencies = [
  "nih_plug_xtask",
 ]
+
+[[package]]
+name = "zip"
+version = "0.5.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "93ab48844d61251bb3835145c521d88aa4031d7139e8485990f60ca911fa0815"
+dependencies = [
+ "byteorder",
+ "bzip2",
+ "crc32fast",
+ "flate2",
+ "thiserror",
+ "time",
+]

plugins/examples/stft/Cargo.toml

@@ -10,3 +10,5 @@ crate-type = ["cdylib"]
 
 [dependencies]
 nih_plug = { path = "../../../", features = ["assert_process_allocs"] }
+
+fftw = "0.7.0"

plugins/examples/stft/src/lib.rs

@@ -1,3 +1,6 @@
+use fftw::array::AlignedVec;
+use fftw::plan::{C2RPlan, C2RPlan32, R2CPlan, R2CPlan32};
+use fftw::types::{c32, Flag};
 use nih_plug::prelude::*;
 use std::pin::Pin;
 
@@ -7,25 +10,82 @@ const OVERLAP_TIMES: usize = 4;
 struct Stft {
     params: Pin<Box<StftParams>>,
 
+    /// An adapter that performs most of the overlap-add algorithm for us.
     stft: util::StftHelper,
+    /// A Hann window window, passed to the overlap-add helper.
     window_function: Vec<f32>,
+
+    /// The FFT of a simple low pass FIR filter.
+    lp_filter_kernel: Vec<c32>,
+
+    /// The algorithms for the FFT and IFFT operations.
+    plan: Plan,
+    /// Scratch buffers for computing our FFT. The [`StftHelper`] already contains a buffer for the
+    /// real values.
+    complex_fft_scratch_buffer: AlignedVec<c32>,
 }
 
+/// FFTW uses raw pointers which aren't Send+Sync, so we'll wrap this in a separate struct.
+struct Plan {
+    r2c_plan: R2CPlan32,
+    c2r_plan: C2RPlan32,
+}
+
+unsafe impl Send for Plan {}
+unsafe impl Sync for Plan {}
+
 #[derive(Params)]
-#[allow(clippy::derivable_impls)]
 struct StftParams {}
 
 impl Default for Stft {
     fn default() -> Self {
+        let mut r2c_plan: R2CPlan32 = R2CPlan32::aligned(&[WINDOW_SIZE], Flag::MEASURE).unwrap();
+        let c2r_plan: C2RPlan32 = C2RPlan32::aligned(&[WINDOW_SIZE], Flag::MEASURE).unwrap();
+        let mut real_fft_scratch_buffer: AlignedVec<f32> = AlignedVec::new(WINDOW_SIZE);
+        let mut complex_fft_scratch_buffer: AlignedVec<c32> = AlignedVec::new(WINDOW_SIZE / 2 + 1);
+
+        // Build a super simple low pass filter from one of the built in window function
+        const FILTER_WINDOW_SIZE: usize = 33;
+        let filter_window = util::window::hann(FILTER_WINDOW_SIZE);
+        real_fft_scratch_buffer[0..FILTER_WINDOW_SIZE].copy_from_slice(&filter_window);
+
+        // Our STFT functions will have a window function applied, so we need to do the same thing
+        // with this filter
+        let window_function = util::window::hann(WINDOW_SIZE);
+        util::window::multiply_with_window(&mut real_fft_scratch_buffer, &window_function);
+
+        // And make sure to normalize this so convolution sums to 1
+        let filter_sum_recip = real_fft_scratch_buffer.iter().sum::<f32>().recip();
+        for sample in real_fft_scratch_buffer.as_slice_mut() {
+            *sample *= filter_sum_recip;
+        }
+
+        r2c_plan
+            .r2c(
+                &mut real_fft_scratch_buffer,
+                &mut complex_fft_scratch_buffer,
+            )
+            .unwrap();
+
         Self {
             params: Box::pin(StftParams::default()),
 
             stft: util::StftHelper::new(2, WINDOW_SIZE),
-            window_function: util::window::hann(WINDOW_SIZE),
+            window_function,
+
+            lp_filter_kernel: complex_fft_scratch_buffer
+                .iter()
+                .take(WINDOW_SIZE)
+                .copied()
+                .collect(),
+
+            plan: Plan { r2c_plan, c2r_plan },
+            complex_fft_scratch_buffer,
         }
     }
 }
 
+#[allow(clippy::derivable_impls)]
 impl Default for StftParams {
     fn default() -> Self {
         Self {}
@@ -73,18 +133,43 @@ impl Plugin for Stft {
         buffer: &mut Buffer,
         _context: &mut impl ProcessContext,
     ) -> ProcessStatus {
-        const GAIN_COMPENSATION: f32 = 2.0 / OVERLAP_TIMES as f32;
+        const GAIN_COMPENSATION: f32 = 1.0 / OVERLAP_TIMES as f32 / WINDOW_SIZE as f32;
+
         self.stft.process_overlap_add(
             buffer,
             [],
             &self.window_function,
             OVERLAP_TIMES,
-            |_channel_idx, _, block| {
+            |_channel_idx, _, real_fft_scratch_buffer| {
-                for sample in block {
+                // Forward FFT, the helper has already applied window function
-                    // TODO: Use the FFTW bindings and do some STFT operation here instead of
+                self.plan
-                    //       reducing the gain at a 2048 sample latency...
+                    .r2c_plan
-                    *sample *= GAIN_COMPENSATION;
+                    .r2c(
+                        real_fft_scratch_buffer,
+                        &mut self.complex_fft_scratch_buffer,
+                    )
+                    .unwrap();
+
+                // As per the convolution theorem we can simply multiply these two buffers. We'll
+                // also apply the gain compensation at this point.
+                for (fft_bin, kernel_bin) in self
+                    .complex_fft_scratch_buffer
+                    .as_slice_mut()
+                    .iter_mut()
+                    .zip(&self.lp_filter_kernel)
+                {
+                    *fft_bin *= *kernel_bin * GAIN_COMPENSATION;
                 }
+
+                // Inverse FFT back into the scratch buffer. This will be added to a ring buffer
+                // which gets written back to the host at a one block delay.
+                self.plan
+                    .c2r_plan
+                    .c2r(
+                        &mut self.complex_fft_scratch_buffer,
+                        real_fft_scratch_buffer,
+                    )
+                    .unwrap();
             },
         );