// Copyright 2014 Tyler Neely
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

use libc::{self, c_char, c_int, c_void, size_t};
use std::ffi::CString;
use std::mem;
use std::ptr;
use std::slice;


pub type MergeFn = fn(&[u8], Option<&[u8]>, &mut MergeOperands) -> Vec<u8>;

pub struct MergeOperatorCallback {
    pub name: CString,
    pub merge_fn: MergeFn,
}

pub extern "C" fn destructor_callback(raw_cb: *mut c_void) {
    // turn this back into a local variable so rust will reclaim it
    let _: Box<MergeOperatorCallback> = unsafe { mem::transmute(raw_cb) };
}

pub extern "C" fn name_callback(raw_cb: *mut c_void) -> *const c_char {
    unsafe {
        let cb: &mut MergeOperatorCallback = &mut *(raw_cb as *mut MergeOperatorCallback);
        let ptr = cb.name.as_ptr();
        ptr as *const c_char
    }
}

pub extern "C" fn full_merge_callback(
    raw_cb: *mut c_void,
    raw_key: *const c_char,
    key_len: size_t,
    existing_value: *const c_char,
    existing_value_len: size_t,
    operands_list: *const *const c_char,
    operands_list_len: *const size_t,
    num_operands: c_int,
    success: *mut u8,
    new_value_length: *mut size_t,
) -> *const c_char {
    unsafe {
        let cb: &mut MergeOperatorCallback = &mut *(raw_cb as *mut MergeOperatorCallback);
        let operands = &mut MergeOperands::new(operands_list, operands_list_len, num_operands);
        let key: &[u8] = slice::from_raw_parts(raw_key as *const u8, key_len as usize);
        let oldval: &[u8] =
            slice::from_raw_parts(existing_value as *const u8, existing_value_len as usize);
        let mut result = (cb.merge_fn)(key, Some(oldval), operands);
        result.shrink_to_fit();
        // TODO(tan) investigate zero-copy techniques to improve performance
        let buf = libc::malloc(result.len() as size_t);
        assert!(!buf.is_null());
        *new_value_length = result.len() as size_t;
        *success = 1 as u8;
        ptr::copy(result.as_ptr() as *mut c_void, &mut *buf, result.len());
        buf as *const c_char
    }
}

pub extern "C" fn partial_merge_callback(
    raw_cb: *mut c_void,
    raw_key: *const c_char,
    key_len: size_t,
    operands_list: *const *const c_char,
    operands_list_len: *const size_t,
    num_operands: c_int,
    success: *mut u8,
    new_value_length: *mut size_t,
) -> *const c_char {
    unsafe {
        let cb: &mut MergeOperatorCallback = &mut *(raw_cb as *mut MergeOperatorCallback);
        let operands = &mut MergeOperands::new(operands_list, operands_list_len, num_operands);
        let key: &[u8] = slice::from_raw_parts(raw_key as *const u8, key_len as usize);
        let mut result = (cb.merge_fn)(key, None, operands);
        result.shrink_to_fit();
        // TODO(tan) investigate zero-copy techniques to improve performance
        let buf = libc::malloc(result.len() as size_t);
        assert!(!buf.is_null());
        *new_value_length = 1 as size_t;
        *success = 1 as u8;
        ptr::copy(result.as_ptr() as *mut c_void, &mut *buf, result.len());
        buf as *const c_char
    }
}


pub struct MergeOperands {
    operands_list: *const *const c_char,
    operands_list_len: *const size_t,
    num_operands: usize,
    cursor: usize,
}

impl MergeOperands {
    fn new(
        operands_list: *const *const c_char,
        operands_list_len: *const size_t,
        num_operands: c_int,
    ) -> MergeOperands {
        assert!(num_operands >= 0);
        MergeOperands {
            operands_list: operands_list,
            operands_list_len: operands_list_len,
            num_operands: num_operands as usize,
            cursor: 0,
        }
    }
}

impl<'a> Iterator for &'a mut MergeOperands {
    type Item = &'a [u8];
    fn next(&mut self) -> Option<&'a [u8]> {
        if self.cursor == self.num_operands {
            None
        } else {
            unsafe {
                let base = self.operands_list as usize;
                let base_len = self.operands_list_len as usize;
                let spacing = mem::size_of::<*const *const u8>();
                let spacing_len = mem::size_of::<*const size_t>();
                let len_ptr = (base_len + (spacing_len * self.cursor)) as *const size_t;
                let len = *len_ptr as usize;
                let ptr = base + (spacing * self.cursor);
                self.cursor += 1;
                Some(mem::transmute(slice::from_raw_parts(
                    *(ptr as *const *const u8) as *const u8,
                    len,
                )))
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.num_operands - self.cursor;
        (remaining, Some(remaining))
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use rocksdb::{DBVector, Writable, DB};
    use rocksdb_options::{ColumnFamilyOptions, DBOptions};
    use tempdir::TempDir;

    #[allow(unused_variables)]
    #[allow(dead_code)]
    fn test_provided_merge(
        new_key: &[u8],
        existing_val: Option<&[u8]>,
        operands: &mut MergeOperands,
    ) -> Vec<u8> {
        let nops = operands.size_hint().0;
        let mut result: Vec<u8> = Vec::with_capacity(nops);
        if let Some(v) = existing_val {
            for e in v {
                result.push(*e);
            }
        }
        for op in operands {
            for e in op {
                result.push(*e);
            }
        }
        result
    }

    #[allow(dead_code)]
    #[test]
    fn mergetest() {
        let path = TempDir::new("_rust_rocksdb_mergetest").expect("");
        let mut opts = DBOptions::new();
        opts.create_if_missing(true);
        let mut cf_opts = ColumnFamilyOptions::new();
        cf_opts.add_merge_operator("test operator", test_provided_merge);
        let db = DB::open_cf(
            opts,
            path.path().to_str().unwrap(),
            vec![("default", cf_opts)],
        ).unwrap();
        let p = db.put(b"k1", b"a");
        assert!(p.is_ok());
        let _ = db.merge(b"k1", b"b");
        let _ = db.merge(b"k1", b"c");
        let _ = db.merge(b"k1", b"d");
        let _ = db.merge(b"k1", b"efg");
        let m = db.merge(b"k1", b"h");
        assert!(m.is_ok());
        match db.get(b"k1") {
            Ok(Some(value)) => match value.to_utf8() {
                Some(v) => println!("retrieved utf8 value: {}", v),
                None => println!("did not read valid utf-8 out of the db"),
            },
            Err(e) => println!("error reading value {:?}", e),
            _ => panic!("value not present"),
        }

        assert!(m.is_ok());
        let r: Result<Option<DBVector>, String> = db.get(b"k1");
        assert!(r.unwrap().unwrap().to_utf8().unwrap() == "abcdefgh");
        assert!(db.delete(b"k1").is_ok());
        assert!(db.get(b"k1").unwrap().is_none());
    }
}
