add load_string and save_string apis.

src/booster.rs

@@ -1,5 +1,6 @@
 use libc::{c_char, c_double, c_longlong, c_void};
 use std;
+use std::convert::TryInto;
 use std::ffi::CString;
 
 use serde_json::Value;
@@ -32,6 +33,20 @@ impl Booster {
         Ok(Booster::new(handle))
     }
 
+    /// Init from model string.
+    pub fn from_string(model_description: &str) -> Result<Self> {
+        let cstring = CString::new(model_description).unwrap();
+        let mut out_num_iterations = 0;
+        let mut handle = std::ptr::null_mut();
+        lgbm_call!(lightgbm_sys::LGBM_BoosterLoadModelFromString(
+            cstring.as_ptr() as *const c_char,
+            &mut out_num_iterations,
+            &mut handle
+        ))?;
+
+        Ok(Booster::new(handle))
+    }
+
     /// Create a new Booster model with given Dataset and parameters.
     ///
     /// Example
@@ -210,6 +225,48 @@ impl Booster {
         ))?;
         Ok(())
     }
+
+    /// Save model to string. This returns the same content that `save_file` writes into a file.
+    pub fn save_string(&self) -> Result<String> {
+        // get nessesary buffer size
+        let mut out_size = 0_i64;
+        lgbm_call!(lightgbm_sys::LGBM_BoosterSaveModelToString(
+            self.handle,
+            0_i32,
+            -1_i32,
+            0_i32,
+            0,
+            &mut out_size as *mut _,
+            std::ptr::null_mut() as *mut i8
+        ))?;
+
+        // write data to buffer and convert
+        let mut buffer = vec![
+            0u8;
+            out_size
+                .try_into()
+                .map_err(|_| Error::new("size negative"))?
+        ];
+        lgbm_call!(lightgbm_sys::LGBM_BoosterSaveModelToString(
+            self.handle,
+            0_i32,
+            -1_i32,
+            0_i32,
+            buffer.len() as c_longlong,
+            &mut out_size as *mut _,
+            buffer.as_mut_ptr() as *mut c_char
+        ))?;
+
+        if buffer.pop() != Some(0) {
+            // this should never happen, unless lightgbm has a bug
+            panic!("write out of bounds happened in lightgbm call");
+        }
+
+        let cstring = CString::new(buffer).map_err(|e| Error::new(e.to_string()))?;
+        cstring
+            .into_string()
+            .map_err(|_| Error::new("can't convert model string to unicode"))
+    }
 }
 
 impl Drop for Booster {
@@ -300,8 +357,28 @@ mod tests {
         let _ = fs::remove_file("./test/test_save_file.output");
     }
 
+    #[test]
+    fn save_string() {
+        let params = _default_params();
+        let bst = _train_booster(&params);
+        let filename = "./test/test_save_string.output";
+        assert_eq!(bst.save_file(&filename), Ok(()));
+        assert!(Path::new(&filename).exists());
+        let booster_file_content = fs::read_to_string(&filename).unwrap();
+        let _ = fs::remove_file("./test/test_save_file.output");
+
+        assert!(!booster_file_content.is_empty());
+        assert_eq!(Ok(booster_file_content), bst.save_string())
+    }
+
     #[test]
     fn from_file() {
         let _ = Booster::from_file(&"./test/test_from_file.input");
     }
+
+    #[test]
+    fn from_string() {
+        let model_string = fs::read_to_string("./test/test_from_file.input").unwrap();
+        Booster::from_string(&model_string).unwrap();
+    }
 }