  Observational surveys are now able to detect an increasing number of
transients, such as core-collapse supernovae (SN) and powerful non-terminal
outbursts (SN impostors). Dedicated spectroscopic facilities can follow up
these events shortly after detection. Here we investigate the properties of
these explosions at early times. We use the radiative transfer code CMFGEN to
build an extensive library of spectra simulating the interaction of supernovae
and their progenitor's winds/circumstellar medium (CSM). We consider a range of
progenitor mass-loss rates (Mdot = 5e-4 to 1e-2 Msun/yr), abundances (solar,
CNO-processed, and He-rich), and SN luminosities (L = 1.9e8 to 2.5e10 Lsun).
The models simulate events ~1 day after explosion, and we assume a fixed
location of the shock front as Rin=8.6e13 cm. We show that the large range of
massive star properties at the pre-SN stage causes a diversity of early-time
interacting SN and impostors. We identify three main classes of early-time
spectra consisting of relatively high-ionisation (e.g. Ovi), medium-ionisation
(e.g. Ciii), and low-ionisation lines (e.g. Feii/iii). They are regulated by L
and the CSM density. Given a progenitor wind velocity Vinf, our models also
place a lower limit of Mdot > 5e-4 (Vinf/150 km/s) Msun/yr for detection of CSM
interaction signatures in observed spectra. Early-time SN spectra should
provide clear constraints on progenitors by measuring H, He, and CNO abundances
if the progenitors come from single stars. The connections are less clear
considering the effects of binary evolution. Yet, our models provide a clear
path for linking the final stages of massive stars to their post-explosion
spectra at early times, and guiding future observational follow-up of
transients with facilities such as the Zwicky Transient Facility.
