colinmarc commented on code in PR #3574:
URL: https://github.com/apache/iceberg-rust/pull/3574#discussion_r4423203939


##########
crates/iceberg/src/spec/values/datum.rs:
##########
@@ -1312,3 +1306,104 @@ fn human_float<T: Float + LowerExp + FromStr>(value: T) 
-> String {
         format!("{sign}{}.{fraction}E{exponent}", &digits[..1])
     }
 }
+
+const MICROS_PER_DAY: i64 = 24 * 60 * 60 * 1_000_000;
+const NANOS_PER_MICRO: i64 = 1000;
+const NANOS_PER_DAY: i64 = NANOS_PER_MICRO * MICROS_PER_DAY;
+
+/// Converts an int literal between two [PrimitiveType]s.
+fn convert_int(
+    val: i32,
+    source_type: &PrimitiveType,
+    target_type: &PrimitiveType,
+) -> Result<Datum> {
+    let datum = match (source_type, target_type) {
+        (_, PrimitiveType::Int) => Datum::int(val),
+        (_, PrimitiveType::Date) => Datum::date(val),
+        (_, PrimitiveType::Long) => Datum::long(val),
+        (PrimitiveType::Date, PrimitiveType::Timestamp) => 
Datum::timestamp_micros(
+            (val as i64)
+                .checked_mul(MICROS_PER_DAY)
+                .ok_or_else(overflow)?,
+        ),
+        (PrimitiveType::Date, PrimitiveType::Timestamptz) => 
Datum::timestamptz_micros(
+            (val as i64)
+                .checked_mul(MICROS_PER_DAY)
+                .ok_or_else(overflow)?,
+        ),
+        (PrimitiveType::Date, PrimitiveType::TimestampNs) => 
Datum::timestamp_nanos(
+            (val as i64)
+                .checked_mul(NANOS_PER_DAY)
+                .ok_or_else(overflow)?,
+        ),
+        (PrimitiveType::Date, PrimitiveType::TimestamptzNs) => 
Datum::timestamptz_nanos(
+            (val as i64)
+                .checked_mul(NANOS_PER_DAY)
+                .ok_or_else(overflow)?,
+        ),
+        _ => {
+            return Err(invalid_data!(
+                "Can't convert datum from {source_type} type to {target_type} 
type."
+            ));
+        }
+    };
+
+    Ok(datum)
+}
+
+/// Converts a long literal between two [PrimitiveType]s.
+fn convert_long(
+    val: i64,
+    source_type: &PrimitiveType,
+    target_type: &PrimitiveType,
+) -> Result<Datum> {
+    let datum = match (source_type, target_type) {
+        (_, PrimitiveType::Int) => Datum::i64_to_i32(val),
+        (_, PrimitiveType::Long) => Datum::long(val),
+        (
+            PrimitiveType::Long | PrimitiveType::Timestamp | 
PrimitiveType::Timestamptz,
+            PrimitiveType::Timestamp,
+        ) => Datum::timestamp_micros(val),
+        (
+            PrimitiveType::Long | PrimitiveType::Timestamp | 
PrimitiveType::Timestamptz,
+            PrimitiveType::Timestamptz,
+        ) => Datum::timestamptz_micros(val),
+        (
+            PrimitiveType::Long | PrimitiveType::TimestampNs | 
PrimitiveType::TimestamptzNs,
+            PrimitiveType::TimestampNs,
+        ) => Datum::timestamp_nanos(val),
+        (
+            PrimitiveType::Long | PrimitiveType::TimestampNs | 
PrimitiveType::TimestamptzNs,
+            PrimitiveType::TimestamptzNs,
+        ) => Datum::timestamptz_nanos(val),
+        (PrimitiveType::TimestampNs | PrimitiveType::TimestamptzNs, 
PrimitiveType::Timestamp) => {
+            Datum::timestamp_micros(val.div_euclid(NANOS_PER_MICRO))
+        }
+        (PrimitiveType::TimestampNs | PrimitiveType::TimestamptzNs, 
PrimitiveType::Timestamptz) => {
+            Datum::timestamptz_micros(val.div_euclid(NANOS_PER_MICRO))
+        }
+        (PrimitiveType::Timestamp | PrimitiveType::Timestamptz, 
PrimitiveType::TimestampNs) => {
+            
Datum::timestamp_nanos(val.checked_mul(NANOS_PER_MICRO).ok_or_else(overflow)?)
+        }
+        (PrimitiveType::Timestamp | PrimitiveType::Timestamptz, 
PrimitiveType::TimestamptzNs) => {
+            
Datum::timestamptz_nanos(val.checked_mul(NANOS_PER_MICRO).ok_or_else(overflow)?)
+        }
+        (PrimitiveType::Timestamp | PrimitiveType::Timestamptz, 
PrimitiveType::Date) => {
+            Datum::date(val.div_euclid(MICROS_PER_DAY) as i32)
+        }
+        (PrimitiveType::TimestampNs | PrimitiveType::TimestamptzNs, 
PrimitiveType::Date) => {
+            Datum::date(val.div_euclid(NANOS_PER_DAY) as i32)
+        }

Review Comment:
   Seems much better to fail than to do a lossy conversion, and whether to 
deviate from the Java impl is your call :)
   
   I agree that this is not as critical if 
https://github.com/apache/datafusion-iceberg/pull/91 lands, though.



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