added functionality for setting line offset

src/config.rs

@@ -92,6 +92,10 @@ pub struct Config {
     #[structopt(long, value_name = "PAD", default_value = "2")]
     pub line_pad: u32,
 
+    /// Line number offset
+    #[structopt(long, value_name = "OFFSET", default_value = "1")]
+    pub line_offset: u32,
+
     /// List all themes.
     #[structopt(long)]
     pub list_themes: bool,
@@ -231,7 +235,8 @@ impl Config {
             .window_controls(!self.no_window_controls)
             .shadow_adder(self.get_shadow_adder()?)
             .tab_width(self.tab_width)
-            .highlight_lines(self.highlight_lines.clone().unwrap_or_default());
+            .highlight_lines(self.highlight_lines.clone().unwrap_or_default())
+            .line_offset(self.line_offset);
 
         Ok(formatter.build()?)
     }

src/formatter.rs

@@ -36,6 +36,8 @@ pub struct ImageFormatter {
     shadow_adder: Option<ShadowAdder>,
     /// Tab width
     tab_width: u8,
+    /// Line Offset
+    line_offset: u32,
 }
 
 #[derive(Default)]
@@ -56,6 +58,8 @@ pub struct ImageFormatterBuilder<S> {
     shadow_adder: Option<ShadowAdder>,
     /// Tab width
     tab_width: u8,
+    /// Line Offset
+    line_offset: u32,
 }
 
 // FIXME: cannot use `ImageFormatterBuilder::new().build()` bacuse cannot infer type for `S`
@@ -77,6 +81,12 @@ impl<S: AsRef<str> + Default> ImageFormatterBuilder<S> {
         self
     }
 
+    /// Set Line offset
+    pub fn line_offset(mut self, offset: u32) -> Self {
+        self.line_offset = offset;
+        self
+    }
+
     /// Set the pad between lines
     pub fn line_pad(mut self, pad: u32) -> Self {
         self.line_pad = pad;
@@ -140,6 +150,7 @@ impl<S: AsRef<str> + Default> ImageFormatterBuilder<S> {
             tab_width: self.tab_width,
             code_pad_top,
             font,
+            line_offset: self.line_offset,
         })
     }
 }
@@ -227,7 +238,11 @@ impl ImageFormatter {
             *i = (*i).saturating_sub(20);
         }
         for i in 0..=lineno {
-            let line_mumber = format!("{:>width$}", i + 1, width = self.line_number_chars as usize);
+            let line_mumber = format!(
+                "{:>width$}",
+                i + self.line_offset,
+                width = self.line_number_chars as usize
+            );
             self.font.draw_text_mut(
                 image,
                 color,
@@ -259,7 +274,8 @@ impl ImageFormatter {
     // TODO: use &T instead of &mut T ?
     pub fn format(&mut self, v: &[Vec<(Style, &str)>], theme: &Theme) -> DynamicImage {
         if self.line_number {
-            self.line_number_chars = ((v.len() as f32).log10() + 1.0).floor() as u32;
+            self.line_number_chars =
+                (((v.len() + self.line_offset as usize) as f32).log10() + 1.0).floor() as u32;
         } else {
             self.line_number_chars = 0;
             self.line_number_pad = 0;