From 19030b8c6a7cde7d8f1f185b6a3f7a426cc33448 Mon Sep 17 00:00:00 2001
From: Andrew <andrew@digerati.co.nz>
Date: Sat, 5 Nov 2022 14:48:24 +1300
Subject: [PATCH] Add brute filter

---
 src/filters.rs |  4 +++-
 src/png/mod.rs | 31 +++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/src/filters.rs b/src/filters.rs
index 5cf84eda..ec62e8fe 100644
--- a/src/filters.rs
+++ b/src/filters.rs
@@ -16,6 +16,7 @@ pub enum RowFilter {
     Entropy,
     Bigrams,
     BigEnt,
+    Brute,
 }
 
 impl TryFrom<u8> for RowFilter {
@@ -44,13 +45,14 @@ impl Display for RowFilter {
                 Self::Entropy => "Entropy",
                 Self::Bigrams => "Bigrams",
                 Self::BigEnt => "BigEnt",
+                Self::Brute => "Brute",
             }
         )
     }
 }
 
 impl RowFilter {
-    pub const LAST: u8 = Self::BigEnt as u8;
+    pub const LAST: u8 = Self::Brute as u8;
     pub const STANDARD: [Self; 5] = [Self::None, Self::Sub, Self::Up, Self::Average, Self::Paeth];
     pub const SINGLE_LINE: [Self; 2] = [Self::None, Self::Sub];
 
diff --git a/src/png/mod.rs b/src/png/mod.rs
index 2867d997..30668eea 100644
--- a/src/png/mod.rs
+++ b/src/png/mod.rs
@@ -6,6 +6,7 @@ use crate::headers::*;
 use crate::interlace::{deinterlace_image, interlace_image};
 use bitvec::bitarr;
 use indexmap::IndexMap;
+use libdeflater::{CompressionLvl, Compressor};
 use rgb::ComponentSlice;
 use rgb::RGBA8;
 use rustc_hash::FxHashMap;
@@ -19,6 +20,11 @@ pub(crate) mod scan_lines;
 
 use self::scan_lines::{ScanLines, ScanLinesMut};
 
+/// Compression level to use for the Brute filter strategy
+const BRUTE_LEVEL: i32 = 1; // 1 is fastest, 2-4 are not useful, 5 is slower but more effective
+/// Number of lines to compress with the Brute filter strategy
+const BRUTE_LINES: usize = 4; // Values over 8 are generally not useful
+
 #[derive(Debug, Clone)]
 pub struct PngImage {
     /// The headers stored in the IHDR chunk
@@ -406,6 +412,31 @@ impl PngImage {
                             }
                         }
                     }
+                    RowFilter::Brute => {
+                        // Brute force by compressing each filter attempt
+                        // Similar to that of LodePNG but includes some previous lines for context
+                        let mut best_size = usize::MAX;
+                        let line_start = filtered.len();
+                        filtered.resize(filtered.len() + line.data.len() + 1, 0);
+                        let mut compressor =
+                            Compressor::new(CompressionLvl::new(BRUTE_LEVEL).unwrap());
+                        let limit = filtered.len().min((line.data.len() + 1) * BRUTE_LINES);
+                        let capacity = compressor.zlib_compress_bound(limit);
+                        let mut dest = vec![0; capacity];
+
+                        for try_filter in try_filters {
+                            try_filter.filter_line(bpp, line.data, last_line, &mut f_buf);
+                            filtered[line_start..].copy_from_slice(&f_buf);
+                            let size = compressor
+                                .zlib_compress(&filtered[filtered.len() - limit..], &mut dest)
+                                .unwrap_or(usize::MAX);
+                            if size < best_size {
+                                best_size = size;
+                                std::mem::swap(&mut best_line, &mut f_buf);
+                            }
+                        }
+                        filtered.resize(line_start, 0);
+                    }
                     _ => unreachable!(),
                 }
                 filtered.extend_from_slice(&best_line);