feat: 搭起下载流程的框架，待填充具体下载过程

src/core/status.rs

@@ -0,0 +1,62 @@
+use crate::Result;
+
+static STATUS_MAX_RETRY: u32 = 0b100;
+static STATUS_OK: u32 = 0b111;
+
+/// 用来表示下载的状态，不想写太多列了，所以仅使用一个 u32 表示
+/// 从低位开始，固定每三位表示一种数据的状态
+pub struct Status(u32);
+
+impl Status {
+    pub fn new(status: u32) -> Self {
+        Self(status)
+    }
+
+    pub fn should_run(&self) -> [bool; 4] {
+        let mut result = [false; 4];
+        for (i, res) in result.iter_mut().enumerate() {
+            *res = self.check_continue(i);
+        }
+        result
+    }
+
+    pub fn update_status(&mut self, result: &[Result<()>]) {
+        assert!(result.len() >= 4, "result length must be 4");
+        for (i, res) in result.iter().enumerate().take(4) {
+            self.set_result(res, i);
+        }
+        if self.should_run().iter().all(|x| !x) {
+            // 所有任务都成功或者由于尝试次数过多失败，为 status 最高位打上标记，将来不再重试
+            self.0 |= 1 << 31;
+        }
+    }
+
+    fn check_continue(&self, offset: usize) -> bool {
+        assert!(offset < 10, "u32 can only store 10 status");
+        let helper = !0u32;
+        let sub_status = self.0 & (helper << (offset * 3)) & (helper >> (32 - 3 * offset - 3));
+        sub_status < STATUS_MAX_RETRY
+    }
+
+    fn set_result(&mut self, result: &Result<()>, offset: usize) {
+        if result.is_ok() {
+            self.set_ok(offset);
+        } else {
+            self.plus_one(offset);
+        }
+    }
+
+    fn plus_one(&mut self, offset: usize) {
+        self.0 += 1 << (3 * offset);
+    }
+
+    fn set_ok(&mut self, offset: usize) {
+        self.0 |= STATUS_OK << (3 * offset);
+    }
+}
+
+impl From<Status> for u32 {
+    fn from(status: Status) -> Self {
+        status.0
+    }
+}